-
Notifications
You must be signed in to change notification settings - Fork 0
/
glossary.html
1206 lines (1193 loc) · 94.7 KB
/
glossary.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="description" content="The site is intended as a free educational resource about the frontiers of galaxy formation." />
<meta name="keywords" content="glossary, astronomy, astronomy vocab, vocabulary, early universe, cosmic dawn, first galaxies, first stars, population 3, cosmology" />
<meta name="author" content="Erika Hoffman" />
<title>Glossary - Cosmic Dark to Cosmic Dawn</title>
<link rel="stylesheet" type="text/css" href="style.css" />
<link rel="icon" href="images/galaxy_icon_d.png">
</head>
<body>
<div class="grid_1">
<! TOP NAVIGATION !>
<header>
<nav class="navbar" style="grid-row:1;">
<a href="index.html"><h1 class="main-header">Cosmic Dark to Cosmic Dawn</h1></a>
</nav>
<nav class="navbar" style="grid-row:1;">
<ul style="padding-top: 27px; ">
<li><a href="dark_ages.html">What happened during the <span class="bold">Dark Ages?</span></a></li>
<li><a href="cosmic_dawn_1.html">How did the <span class="bold">first stars</span> form?</a></li>
<li><a href="cosmic_dawn_2.html">How did the <span class="bold">first galaxies</span> form?</a></li>
<li><a href="observations.html">How do we <span class="bold">observe</span> them?</a></li>
</ul>
</nav>
<div class="glos">
<ul>
<li>
<! THE DROPDOWN MENU !>
<div class="dropdown">
<button class="dropbtn">
Menu
</button>
<div class="dropdown-content">
<a href="index.html">Home</a>
<a href="dark_ages.html" class="dark_head">The Dark Ages</a>
<a href="big_bang_cmb.html" class="dark_cont">The Big Bang</a>
<a href="cosmic_web.html" class="dark_cont">The Cosmic Web</a>
<a href="exotic_physics.html" class="dark_cont">Exotic Physics</a>
<a href="cosmic_dawn_1.html" class="cd_1_head">Era of the First Stars</a>
<a href="first_stars.html" class="cd_1_cont">The First Stars</a>
<a href="first_black_holes.html" class="cd_1_cont">The First Black Holes</a>
<a href="light_fills_the_universe.html" class="cd_1_cont">Light Fills the Universe</a>
<a href="spin_flip.html" class="cd_1_cont">The Spin-Flip Backgound</a>
<a href="cosmic_dawn_2.html" class="cd_2_head">Era of First Galaxies</a>
<a href="first_galaxies.html" class="cd_2_cont">The First Galaxies</a>
<a href="larger_galaxies.html" class="cd_2_cont">Larger and Larger Galaxies</a>
<a href="epoch_of_reionization.html" class="cd_2_cont">Epoch of Reionization</a>
<a href="later_universe.html" class="later_head">The Later Universe</a>
<a href="cosmic_noon.html" class="later_cont">Galaxies at Cosmic Noon</a>
<a href="our_galaxy.html" class="later_cont">Our Galaxy: The Milky Way</a>
<a href="observations.html" class="obs_head">Obervations Overview</a>
<a href="radio_telescopes.html" class="obs_cont">Radio Telescopes</a>
<a href="lunar_telescopes.html" class="obs_cont">Lunar Radio Telescopes</a>
<a href="space_telescopes.html" class="obs_cont">Space Telescopes</a>
<a href="infrared_telescopes.html" class="obs_cont">Ground-Based Near-Infrared Telescopes</a>
<a href="other_telescopes.html" class="obs_cont">Other Telescopes</a>
</div>
</div>
</li>
<li>
<div class="dropdown">
<button class="dropbtn">Research</button>
<div class="dropdown-content">
<a href="research.html">Research Overview</a>
<a href="group_members.html">Group Members</a>
<a href="collaborations.html">Collaborations</a>
<a href="codes_and_tools.html">Codes and Tools</a>
</div>
</div>
</li>
<li><a href="glossary.html" class="dropbtn">Glossary</a></li>
</ul>
</div>
</header>
</div>
<div class="gloss_title">GLOSSARY</div>
<div>
<ul class="glossary_list">
<li>
<! Annihilation !>
<div class="glos_def">
<span class="bold">Annihilation (of particles):</span> Particle annihilation occurs when a subatomic particle and its respective antiparticle collide, causing both to disappear and release energy. <br />
<span class="img_cred_body">Pages: <a href="big_bang_cmb.html">Big Bang</a>, <a href="exotic_physics.html">Exotic Physics</a></span>
</div>
<div class="glos_img_cred">
<br />Image: A diagram of particle annihilation.
</div>
<div class="glos_img" style="background-image: url(images/annihilation_diagram.png); background-size:45%;">
</div>
</li>
<li>
<! Accretion !>
<div class="glos_def">
<span class="bold">Accretion:</span> The process by which a large astronomical body or mass, such as a galaxy or black hole, accumulates more matter through its gravitational attraction. <br />
<span class="img_cred_body">Pages: <a href="first_black_holes.html">First Black Holes</a>, <a href="first_galaxies.html">First Galaxies</a>, <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a></span>
</div>
<div class="glos_img_cred">
Image: This artist's conception shows a supermassive black hole at the center of a remote galaxy digesting the remnants of a star through accretion.
<br />Credit: <a href="https://images.nasa.gov/details-PIA01884" target="_blank" rel="noopener noreferrer">JPL</a>
</div>
<div class="glos_img" style="background-image:url(images/hungry_black_hole.jpg); background-size:80%;">
</div>
</li>
<li>
<! Accretion DISK !>
<div class="glos_def">
<span class="bold">Accretion Disk:</span> The matter infalling on a dense, rotating object, often a black hole, typically forms a thin disk. Inside the disk, material spirals into the central object.<br />
<span class="img_cred_body">Pages: <a href="first_black_holes.html">First Black Holes</a></span>
</div>
<div class="glos_img_cred">
Image: Artist's conception of a bright black hole <i>accretion disk</i>.
<br />Credit: <a href="https://images.nasa.gov/details-PIA20912" target="_blank" rel="noopener noreferrer">JPL</a>
</div>
<div class="glos_img" style="background-image:url(images/accretion_disk.jpg); background-size:80%;">
</div>
</li>
<li>
<! ATMOSPHERIC DISTORTION !>
<div class="glos_def">
<span class="bold">Atmospheric Distortion:</span> As light travels through our atmosphere, it encounters the molecules and other particles. Sometimes, the light bounces off these particles, heading in a different direction. This is a problem for astronomers, because we would like to trace each light wave back to its source. Adaptive optics is one way to correct these distortions, but of course another solution is to avoid the atmosphere entirely by placing the telescope in space!<br />
<span class="img_cred_body">Pages: <a href="space_telescopes.html">Space Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: Transmission and absorption in Earth atmosphere.
Credit: <a href="https://www.researchgate.net/figure/Transmission-and-absorption-in-Earth-atmosphere-source-NASA_fig1_215681384" target="_blank" rel="noopener noreferrer">NASA</a>
</div>
<div class="glos_img" style="background-image:url(images/atm_absorption.png); background-size:105%; background-position-y:100%;"></div>
</li>
<li>
<! ADAPTIVE OPTICS !>
<div class="glos_def">
<span class="bold">Adaptive Optics:</span> A telescope system that compensates for blurring by Earth's atmosphere by bending or flexing the mirror in ways that cancel out the atmosphere's effect.<br />
<span class="img_cred_body">Pages: <a href="infrared_telescopes.html">Near-Infrared Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: Keck's Laser Guide Star System to measure changes in the atmosphere for the adaptive optics system.
<br />Credit: <a href="https://keckobservatory.org/media/keck-i-and-keck-ii/" target="_blank" rel="noopener noreferrer">W. M. Keck Observatory</a>
</div>
<div class="glos_img" style="background-image:url(images/keck_laser_guy.jpg); background-size:40%;">
</div>
</li>
<li>
<! ALMA !>
<div class="glos_def">
<span class="bold">ALMA (Atacama Large Millimeter Array):</span> A large observatory in the Atacama Desert of Chile, which observes radio waves with millimeter wavelengths. It has a huge array of science objectives, including observations of gas and dust in galaxies during the Cosmic Dawn.<br />
<span class="img_cred_body">Pages: <a href="other_telescopes.html">Other Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: ALMA Observatory
<br />Credit: <a href="https://www.almaobservatory.org/en/image-gallery/?images-tags=antennas&pg=2" target="_blank" rel="noopener noreferrer"> © EFE/Ariel Marinkovic</a>
</div>
<div class="glos_img" style="background-image:url(images/almajpg.jpg); background-size:60%;"></div>
</li>
<li>
<! ATHENA !>
<div class="glos_def">
<span class="bold">ATHENA:</span> A planned space telescope that hopes to measure the X-rays produced as black holes swallow nearby gas in distant galaxies (amongst other things). X-ray telescopes must be placed in space, because the Earth's atmosphere absorbs X-rays.<br />
<span class="img_cred_body">Pages: <a href="other_telescopes.html">Other Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: Athena space observatory. Mirror of the telescope located upfront, science payload located opposite.
<br />Credit: <a href="https://www.the-athena-x-ray-observatory.eu/resources/gallery/mission.html" target="_blank" rel="noopener noreferrer">X-IFU Consortium © DB/X-IFU</a>
</div>
<div class="glos_img" style="background-image:url(images/athena.png); "></div>
</li>
<li>
<! Angular Resolution !>
<div class="glos_def">
<span class="bold">Angular Resolution:</span> The ability to distinguish features in an image, or the smallest angle between two objects for which those objects can be seen as separate.<br />
<span class="img_cred_body">Pages: <a href="radio_telescopes.html">Radio Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: A diagram showing that the further an object, the smaller the angular size/extent, highlighting the need for good angular resolution in astronomy.
<br />Credit:<a href="https://imagine.gsfc.nasa.gov/educators/programs/fermi/classroom/agn_guide.html">NASA Fermi Classroom</a>
</div>
<div class="glos_img" style="background-image:url(images/angular_extent.png); background-size:contain;"></div>
</li>
<li>
<! BIG BANG !>
<div class="glos_def">
<span class="bold">Big Bang:</span> The Big Bang model describes the initial phase of the Universe's expansion, from which it began in a hot, dense state and then expanded rapidly and cooled. <br />
<span class="img_cred_body">Pages: <a href="big_bang_cmb.html">Big Bang</a></span>
</div>
<div class="glos_img_cred">
Image: Snapshot of an artist's animation of the big bang.
<br />Credit: <a href="https://svs.gsfc.nasa.gov/12656" target="_blank" rel="noopener noreferrer">NASA's Goddard Space Flight Center /CI Lab</a>
</div>
<div class="glos_img" style="background-image:url(images/big_bang_artist.jpg); "></div>
</li>
<li>
<! BLACK HOLES >
<div class="glos_def">
<span class="bold">Black Hole:</span> A collection of matter so dense that nothing, not even light, can escape its pull. Some massive stars end their lives as black holes, once their fuel supply is exhausted. <br />
<span class="img_cred_body">Pages: <a href="exotic_physics.html">Exotic Physics, </a><a href="first_stars.html">First Stars</a>, <a href="light_fills_the_universe.html">Light Fills The Universe</a>, <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a></span>
</div>
<div class="glos_img_cred">
Image: An artist's conception of the most primitive <i>supermassive black holes</i> known, at the core of a young, star-rich galaxy.
<br />Credit: <a href="https://images.nasa.gov/details-PIA12966" target="_blank" rel="noopener noreferrer">JPL</a>
</div>
<div class="glos_img" style="background-image:url(images/black_hole_main.jpg); "></div>
</li>
<li>
<! COSMIC DAWN !>
<div class="glos_def">
<span class="bold">Cosmic Dawn:</span> The first appearance of stars, galaxies, and other luminous objects and structures in the universe. It has not yet been observed but likely occurred a few dozen million years after the Big Bang. <br />
<span class="img_cred_body">Pages: <a href="first_stars.html">First Stars</a>, <a href="light_fills_the_universe.html">Light Fills The Universe</a>, <a href="first_galaxies,html">First Galaxies</a>, <a href="epoch_of_reionization.html">Epoch of Reionization</a></span>
</div>
<div class="glos_img_cred">
Image: A timeline running from the Big Bang (right), tothe present (left). In the middle is the Reionization Period where the initial bubbles caused the cosmic dawn.
<br />Credit: <a href="https://svs.gsfc.nasa.gov/10130">NASA/STScI</a>
</div>
<div class="glos_img" style="background-image:url(images/nasa_dawn.jpg); background-size:contain;"></div>
</li>
<li>
<! CMB !>
<div class="glos_def">
<span class="bold">Cosmic Microwave Background (CMB):</span> After the Big Bang, the Universe was full of high-energy photons. Until about 400,000 years after the Big Bang, these photons interacted strongly with electrons throughout the Universe. But after that time, those electrons combined with protons to form hydrogen atoms. After that, the photons could not interact with matter, and they traveled in straight lines until the present day. During that period, they redshifted so much that they appear as microwaves. The CMB was emitted when the universe had cooled enough to form neutral hydrogen; before this time, the universe was opaque. <br />
<span class="img_cred_body">Pages: <a href="big_bang_cmb.html">Big Bang</a>, <a href="epoch_of_reionization.html">Epoch of Reionization</a>, <a href="other_telescopes.html">Other Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: 13.7 billion year old temperature fluctuations (+/- 200 microKelvin - shown as color differences) that correspond to the seeds that grew to become the galaxies.
<br />Credit: <a href="https://wmap.gsfc.nasa.gov/media/101080/" target="_blank" rel="noopener noreferrer">NASA / WMAP Science Team</a>
</div>
<div class="glos_img" style="background-image:url(images/CMB.png); background-size:contain;"></div>
</li>
<li>
<! COSMIC INFLATION !>
<div class="glos_def">
<span class="bold">Cosmic Inflation:</span> A period of extraordinarily fast expansion very early in the Universe's history which generated tiny fluctuations in the matter density, creating the seeds of galaxies. <br />
<span class="img_cred_body">Pages: <a href="big_bang_cmb.html">Big Bang</a></span>
</div>
<div class="glos_img_cred">
Image: A timeline of our universe, with cosmic inflation as the first big event after the big bang, and when the universe made it's fastest increase in size.
<br />Credit: <a href="https://www.nasa.gov/mission_pages/planck/multimedia/pia16876b.html#.YSWo2o5KjD5">NASA</a>
</div>
<div class="glos_img" style="background-image:url(images/inflation.jpg);"></div>
</li>
<li>
<! COSMIC NOON !>
<div class="glos_def">
<span class="bold">Cosmic Noon:</span> As dark matter clumps grow, their galaxies do as well, forming stars rapidly. This process peaks about three billion years after the Big Bang, a period known as the Cosmic Noon of galaxy formation. <br />
<span class="img_cred_body">Pages: <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a></span>
</div>
<div class="glos_img_cred">
Andromeda galaxy, our closest neighbor.
<br />
Image Credit: <a href=" https://wallpaperaccess.com/nasa-galaxy" target="_blank" rel="noopener noreferrer">NASA Goddard</a>
</div>
<div class="glos_img" style="background-image:url(images/HD_andromeda.jpg); "></div>
</li>
<li>
<! COSMIC RAYS !>
<div class="glos_def">
<span class="bold">Cosmic Rays:</span> High energy particles, such as protons and electrons, that are accelerated near the speed of light by unknown processes. They are common throughout the galaxy, and astronomers suspect many are created following supernova explosions <br />
<span class="img_cred_body">Pages: <a href="radio_telescopes.html">Radio Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: Illistration of Cosmic Rays from Galactic Centers.
<br /> Credit: <a href="https://apod.nasa.gov/apod/ap071112.html" target="_blank" rel="noopener noreferrer">Pierre Auger Observatory Team</a>
</div>
<div class="glos_img" style="background-image:url(images/cosmic_ray.jpg); "></div>
</li>
<li>
<! COSMIC STRINGS !>
<div class="glos_def">
<span class="bold">Cosmic Strings:</span> Vast filament-like structures of very high densities that may have occurred during the Dark Ages early in the Universe's history. Cosmic strings would have formed as relics of high-energy particle physics transitions in the early Universe. <br />
<span class="img_cred_body">Pages: <a href="exotic_physics.html">Exotic Physics</a></span>
</div>
<div class="glos_img_cred">
Image: Snapshot of a cosmic string network during the transition from the radiation epoch to the matter epoch.
<br />Credit: <a href="http://www.ctc.cam.ac.uk/outreach/origins/cosmic_structures_four.php" target="_blank" rel="noopener noreferrer">B. Allen & E.P. Shellard cosmic string simulations</a>
</div>
<div class="glos_img" style="background-image:url(images/cosmic_string.jpg); background-size:110%;"></div>
</li>
<li>
<! COSMIC WEB !>
<div class="glos_def">
<span class="bold">Cosmic Web:</span> The network of sheets, filaments, and voids into which matter in the Universe is organized. Gravitational growth creates asymmetric matter clumps that take the form of a network of intersecting sheets and filaments, separated by large regions of empty space, making the “skeleton” of the universe. <br />
<span class="img_cred_body">Pages: <a href="cosmic_web.html">Cosmic Web</a></span>
</div>
<div class="glos_img_cred">
Image: A snapshot of the cosmic web, from the same simulation as above. This snapshot is at the present day, 13.7 billion years after the Big Bang.
<br />Credit: <a href="https://wwwmpa.mpa-garching.mpg.de/galform/virgo/millennium/" target="_blank" rel="noopener noreferrer">Springel et al. (2005)</a>
</div>
<div class="glos_img" style="background-image:url(images/web_z0.jpg); "></div>
</li>
<li>
<! DAPPER !>
<div class="glos_def">
<span class="bold">DAPPER (Dark Ages Polarimeter Pathfinder):</span> A small lunar-orbiting telescope that would use the Moon as a shield from terrestrial interference to observe the average spin-flip background at several points during the Cosmic Dawn. <br />
<span class="img_cred_body">Pages: <a href="lunar_telescopes.html">Lunar Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: concept of DAPPER on the moon.
<br />Credit: <a href="https://www.colorado.edu/project/dark-ages-polarimeter-pathfinder/" target="_blank" rel="noopener noreferrer">University of Colorado and Tend Studios</a>
</div>
<div class="glos_img" style="background-image:url(images/dapper.jpg); background-size:80%;"></div>
</li>
<li>
<! DARE !>
<div class="glos_def">
<span class="bold">DARE (Dark Ages Radio Explorer):</span> A lunar-orbiting telescope that would use the Moon as a shield from terrestrial interference to observe the spin-flip background during the Cosmic Dawn, over a larger set of times than DAPPER. <br />
<span class="img_cred_body">Pages: <a href="lunar_telescopes.html">Lunar Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: DARE mission concept
<br />Credit: Credit: <a href="https://www.colorado.edu/ness/projects/dark-ages-radio-explorer-dare-mission-concept" target="_blank" rel="noopener noreferrer">NASA NESS, University of Colorado, Boulder</a>
</div>
<div class="glos_img" style="background-image:url(images/dare.jpg); background-size:38%;"></div>
</li>
<li>
<! DARK AGES !>
<div class="glos_def">
<span class="bold">Dark Ages:</span> A time in the universe after the CMB was emitted but before stars or any other objects began producing light, spanning approximately 400,000 years to 50 million years after the Big Bang. <br />
<span class="img_cred_body">Pages: <a href="cosmic_web.html">Cosmic Web</a>, <a href="exotic_physics.html">Exotic Physics</a>, <a href="first_stars.html">First Stars</a>, <a href="epoch_of_reionization.html">Epoch of Reionization</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! DARK MATTER !>
<div class="glos_def">
<span class="bold">Dark Matter:</span> A form of matter that only interacts through gravity – its identity remains a mystery, but its gravity drove the formation of all structure in the Universe. There are many independent lines of evidence for dark matter in astronomy, but it has never been identified in laboratories.<br />
<span class="img_cred_body">Pages: <a href="cosmic_web.html">Cosmic Web</a>, <a href="exotic_physics.html">Exotic Physics</a>, <a href="first_stars.html">First Stars</a>, <a href="first_black_holes.html">First Black Holes</a>, <a href="first_galaxies.html">First Galaxies</a>, <a href="larger_galaxies.html">Larger and Larger Galaxies</a>, <a href="epoch_of_reionization.html">Epoch of Reionization</a></span>
</div>
<div class="glos_img_cred">
Image: Small Magellanic Cloud superimposed with one half of a model of its dark matter (right of center), with lighter colors indicating greater density.
<br />Credit: <a href="https://www.nasa.gov/feature/goddard/2016/nasas-fermi-mission-expands-its-search-for-dark-matter" target="_blank" rel="noopener noreferrer">Dark matter, R. Caputo et al. 2016; background, Axel Mellinger, Central Michigan University"
</div>
<div class="glos_img" style="background-image:url(images/dark_matter.jpg);"></div>
</li>
<li>
<! ELECTRON !>
<div class="glos_def">
<span class="bold">Electron:</span> A subatomic (smaller than an atom) particle that holds negative charge. <br />
<span class="img_cred_body">Pages: <a href="big_bang_cmb.html">Big Bang</a>, <a href="cosmic_web.html">Cosmic Web</a>, <a href="spin_flip.html">Spin Flip Background</a>, <a href="epoch_of_reionization.html">Epoch of Reionization</a>, <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a> </span>
</div>
<div class="glos_img_cred">
Image: Hydrogen, the simpliest atom.
</div>
<div class="glos_img" style="background-image:url(images/electron.png); background-size:contain;"></div>
</li>
<li>
<! ELLIPITICAL GALAXY !>
<div class="glos_def">
<span class="bold">Elliptical Galaxy:</span> A common type of massive galaxy in the nearby Universe. Elliptical galaxies are spherical or (American) football-shaped collections of stars that lack any of the gas that can act as the fuel for future star formation. The origin of this lack of gas is a mystery but it may be connected to their central supermassive black holes. <br />
<span class="img_cred_body">Pages: <a href="epoch_of_reionization.html">Epoch of Reionization</a>, <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! EMISSION LINES !>
<div class="glos_def">
<span class="bold">Emission Lines:</span> Specific wavelengths of radiation emitted by a source with unique energy levels, such as an atom or molecule. In these systems, quantum mechanics demands that the electrons only exist at very specific energies. When an electron drops from one energy level to a lower one, it emits a photon (or particle of light) with a very specific energy and wavelength. When many of these events occur together, astronomers detect many photons with that same wavelength - which appears as an excess of photons at a specific wavelength. <br />
<span class="img_cred_body">Pages: <a href="space_telescopes.html">Space Telescopes</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! EXOPLANET !>
<div class="glos_def">
<span class="bold">Exoplanet:</span> A planet outside of our solar system, orbiting another star. <br />
<span class="img_cred_body">Pages: <a href="lunar_telescopes.html">Lunar Telescopes</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! FARSIDE !>
<div class="glos_def">
<span class="bold">FARSIDE (Farside Array for Radio Science Investigations of the Dark Ages and Exoplanets):</span> A planned mission that would involve the construction of an array of radio antennae on the lunar surface, deployed using robotic rovers. The array would observe the Sun, exoplanets, and eventually the Cosmic Dawn. By placing the array on the far side of the Moon, the Moon itself would block radio waves from Earth - one of the major challenges to observing low-frequency radio waves.<br />
<span class="img_cred_body">Pages: <a href="lunar_telescopes.html">Lunar Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: Concept of FARSIDE: A Low Radio Frequency Interferometric Array on the Lunar Farside
<br />Credit: <a href="https://www.colorado.edu/ness/farside-low-radio-frequency-interferometric-array-lunar-farside" target="_blank" rel="noopener noreferrer">Courtesy of Blue Origin</a>
</div>
<div class="glos_img" style="background-image:url(images/farside.jpg); background-size:contain;"></div>
</li>
<li>
<! FEEDBACK !>
<div class="glos_def">
<span class="bold">Feedback:</span> A key feature of star formation is that the stars themselves are powerful energy sources. They produce an extraordinary amount of light, and some end their lives in massive explosions called supernovae. This energy can disrupt nearby gas clouds, preventing those clouds from forming stars - and sometimes even shoving the gas entirely outside the galaxy! <br />
<span class="img_cred_body">Pages: <a href="light_fills_the_universe.html">Light Fills the Universe</a>, <a href="first_galaxies.html">First Galaxies</a>, <a href="larger_galaxies.html">Larger and Larger Galaxies</a>, <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! GALACTIC BULDGE !>
<div class="glos_def">
<span class="bold">Galactic Buldge:</span> A spherical or (American) football-shaped central region of a spiral galaxy. It contains gas and stars at a variety of ages around a supermassive black hole. <br />
<span class="img_cred_body">Pages: <a href="our_galaxy.html">Our Galaxy: The Milky Way</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! GALACTIC DISK !>
<div class="glos_def">
<span class="bold">Galactic Disk:</span> The thin, pancake-like shape of a spiral galaxy. It contains most of the stars and star-forming gas clouds, as well as the spiral structure.<br />
<span class="img_cred_body">Pages: <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a>, <a href="our_galaxy.html">Our Galaxy: The Milky Way</a></span>
</div>
<div class="glos_img_cred">
Image: The barred <i>spiral galaxy</i> M83, also known as the Southern Pinwheel, 15 million light-years away in the constellation Hydra.
<br />Credit: <a href="https://images.nasa.gov/details-GSFC_20171208_Archive_e001262" target="_blank" rel="noopener noreferrer">NASA, ESA, and the Hubble Heritage Team</a>
</div>
<div class="glos_img" style="background-image:url(images/spiral_galaxy.jpg); "></div>
</li>
<li>
<! GALACTIC HALO !>
<div class="glos_def">
<span class="bold">Galactic Halo:</span> A large spherical region around a galaxy containing a sparse population of very old stars but very little gas. Many of these stars are organized into globular clusters. <br />
<span class="img_cred_body">Pages: <a href="our_galaxy.html">Our Galaxy: The Milky Way</a></span>
</div>
<div class="glos_img_cred">
Image: The barred spiral galaxy M83, also known as the Southern Pinwheel, 15 million light-years away in the constellation Hydra.
<br />Credit: <a href="https://images.nasa.gov/details-GSFC_20171208_Archive_e000794" target="_blank" rel="noopener noreferrer">NASA, ESA, and the Hubble Heritage Team</a>
</div>
<div class="glos_img" style="background-image:url(images/galaxy_halo.jpg); "></div>
</li>
<li>
<! GALAXY !>
<div class="glos_def">
<span class="bold">Galaxy:</span> A discrete group of stars bound together by gravity that is able to form stars over long time periods. Galaxies form inside large clumps of dark matter.<br />
<span class="img_cred_body">Pages: <a href="first_galaxies.html">First Galaxies</a></span>
</div>
<div class="glos_img_cred">
Image: The Andromeda galaxy, the most massive in the Local Group of galaxies (which encompasses our own Milky Way). This galaxy is much bigger than the first structures!
<br />Credit: <a href="https://images.nasa.gov/details-PIA04921" target="_blank" rel="noopener noreferrer">JPL, Caltech, NASA Galaxy Evolution Explorer</a>
</div>
<div class="glos_img" style="background-image:url(images/andromeda_first_gal.jpg); "></div>
</li>
<li>
<! GALAXY CLUSTER !>
<div class="glos_def">
<span class="bold">Galaxy Cluster:</span> A very massive dark matter clump, more than a million billion times the mass of our Sun, that contains thousands of individual galaxies. Like galaxies, galaxy clusters are held together by dark matter. They are the most massive gravitationally-bound objects in the Universe today. <br />
<span class="img_cred_body">Pages: <a href="light_fills_the_universe.html">Light Fills The Universe</a>, <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a></span>
</div>
<div class="glos_img_cred">
Image: An assortment of majestic star cities that vary in age, shape, and stellar populations in <i>galaxy cluster</i> Abell 370.
<br />Credit: <a href="https://images.nasa.gov/details-GSFC_20171208_Archive_e000056" target="_blank" rel="noopener noreferrer">NASA, ESA, J. Lotz and the HFF Team (STScI)</a>
</div>
<div class="glos_img" style="background-image:url(images/cosmic_noon_1.jpg); background-size:contain;"></div>
</li>
<li>
<! GAMMA RAY !>
<div class="glos_def">
<span class="bold">Gamma Ray:</span> High-energy electromagnetic radiation, with the shortest wavelength on the electromagnetic spectrum, less than 10<sup>-12</sup> meters.<br />
<span class="img_cred_body">Pages: <a href="exotic_physics.html">Exotic Physics</a></span>
</div>
<div class="glos_img_cred">
Image: Shows the types of wavelengths going from longest to shortest.
<br />Credit: <a href="https://www.nasa.gov/audience/forstudents/k-4/dictionary/Electromagnetic_Spectrum.html" target="_blank" rel="noopener noreferrer">NASA</a>
</div>
<div class="glos_img" style="background-image:url(images/EM.png); background-size:contain;"></div>
</li>
<li>
<! GLOBULAR CLUSTER !>
<div class="glos_def">
<span class="bold">Globular Clusters:</span> Clumps of about a million old stars packed closely together. Globular clusters contain some of the oldest stars in our Galaxy, but their origin is mysterious.<br />
<span class="img_cred_body">Pages: <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! GRAVITAIONAL WAVE !>
<div class="glos_def">
<span class="bold">Gravitational Wave:</span> Ripples in space-time generated by massive gravitational events, such as the collisions of black holes. Gravitational waves are predicted by Einstein’s General Theory of Relativity.<br />
<span class="img_cred_body">Pages: <a href="other_telescopes.html">Other Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: An artist's impression of gravitational waves generated by binary neutron stars.
<br />Credit: <a href="https://www.nasa.gov/feature/goddard/2016/nsf-s-ligo-has-detected-gravitational-waves" target="_blank" rel="noopener noreferrer">R. Hurt/Caltech-JPL</a>
</div>
<div class="glos_img" style="background-image:url(images/grav_wave.jpg); "></div>
</li>
<li>
<! GRAVITY !>
<div class="glos_def">
<span class="bold">Gravity:</span> A force by which a star or other body draws matter or other objects toward its center. The strength of gravity is proportional to the mass of the objects and decreases as their separation increases.<br />
<span class="img_cred_body">Pages: <a href="cosmic_web.html">Cosmic Web</a>, <a href="exotic_physics.html">Exotic Physics</a>, <a href="first_stars.html">First Stars</a>, <a href="first_black_holes.html">First Black Holes</a>, <a href="first_galaxies.html">First Galaxies</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! HEAVY ELEMENTS !>
<div class="glos_def">
<span class="bold">Heavy Elements:</span> In astronomy, chemical elements with more than two protons are considered heavy - in other words, every element except hydrogen and helium! This definition is useful because hydrogen and helium were created in abundance by the Big Bang, but almost every other element is created much later, inside of stars. Elements with more protons than iron must be created in a supernova, and are considered heavy to astronomers.<br />
<span class="img_cred_body">Pages: <a href="first_galaxies.html">First Galaxies</a>, <a href="larger_galaxies.html">Larger and Larger Galaxies</a>, <a href="our_galaxy.html">Our Galaxy: The Milky Way</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! HERA !>
<div class="glos_def">
<span class="bold">HERA (Hydrogen Epoch of Reionization Array):</span> A large radio telescope in the Karoo Desert of South Africa, and one of the premier radio telescopes studying the Cosmic Dawn. HERA is an interferometer, so it combines signals from an array of individual radio dishes.<br />
<span class="img_cred_body">Pages: <a href="space_telescopes.html">Space Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: Hydrogen Epoch of Reionization Array (HERA) 2016
<br />
Credit: <a href="https://reionization.org" target="_blank" rel="noopener noreferrer">HERA Construction Photo log</a>
</div>
<div class="glos_img" style="background-image:url(images/HERA.jpg); "></div>
</li>
<li>
<! HUBBLE SPACE TELESCOPE !>
<div class="glos_def">
<span class="bold">Hubble Space Telescope:</span> Launched in 1990, this NASA telescope has detected hundreds of galaxies from the first billion years of the Universe’s history and obtained many of the images on this website. The telescope is named after Edwin Hubble, the astronomer who determined that the Universe is bigger than our Galaxy and discovered that the Universe is expanding.<br />
<span class="img_cred_body">Pages: <a href="larger_galaxies.html">Larger and Larger Galaxies</a>, <a href="space_telescopes.html">Space Telescopes</a>, <a href="infrared_telescopes.html">Infrared Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: Hubble Space Telescope in Earth Orbit.
<br />Credit: <a href="https://images.nasa.gov/details-s31-10-035" target="_blank" rel="noopener noreferrer">JSC</a>
</div>
<div class="glos_img" style="background-image:url(images/hst.jpg); "></div>
</li>
<li>
<! HYDROGEN GAS !>
<div class="glos_def">
<span class="bold">Hydrogen Gas:</span> Gas containing hydrogen, the simplest chemical element. Hydrogen consists of a single proton and electron bound together by their electrical attraction.<br />
<span class="img_cred_body">Pages: <a href="light_fills_the_universe.html">Light Fills The Universe</a>, <a href="spin_flip.html">Spin Flip Background</a>, <a href="epoch_of_reionization.html">Epoch of Reionization</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! INFRARED LIGHT !>
<div class="glos_def">
<span class="bold">Infrared Light:</span> Light that is slightly less energetic than visible light, or with a slightly longer wavelength. Because light redshifts as it travels through the Universe, the visible and ultraviolet light produced by galaxies during the Cosmic Dawn is in the infrared range when it reaches us.<br />
<span class="img_cred_body">Pages: <a href="our_galaxy.html">Our Galaxy: The Milky Way</a>, <a href="space_telescopes.html">Space Telescopes</a> </span>
</div>
<div class="glos_img_cred">
Image: Shows the types of wavelengths going from longest to shortest.
<br />Credit: <a href="https://www.nasa.gov/audience/forstudents/k-4/dictionary/Electromagnetic_Spectrum.html" target="_blank" rel="noopener noreferrer">NASA</a>
</div>
<div class="glos_img" style="background-image:url(images/EM.png); background-size:contain;"></div>
</li>
<li>
<! INFRARED TELESCOPES !>
<div class="glos_def">
<span class="bold">Infrared Telescope:</span> Infrared Telescopes measure near-infrared light, rather than the optical light our eyes see, and operate in an analogous way to digital cameras.<br />
<span class="img_cred_body">Pages: <a href="infrared_telescopes.html">Infrared Telescopes</a> </span>
</div>
<div class="glos_img_cred">
Image: The summit of Mauna Kea, Hawaii, with W. M. Keck Observatory.
<br />Credit: <a href="https://images.nasa.gov/details-PIA04494" target="_blank" rel="noopener noreferrer">NASA/JPL</a>
</div>
<div class="glos_img" style="background-image:url(images/keck_nasa.jpg); "></div>
</li>
<li>
<! INTENSITY MAPPING !>
<div class="glos_def">
<span class="bold">Intensity Mapping:</span> A method in which a telescope observes the sky with such blurry vision that it cannot distinguish individual galaxies. Even though the telescope cannot make out individual galaxies in a particular patch of the sky, it can measure all the light coming from them - including the faint ones invisible to larger telescopes.<br />
<span class="img_cred_body">Pages: <a href="other_telescopes.html">Other Telescopes</a> </span>
</div>
<div class="glos_img_cred">
Image: A simulated field with galaxy positions (Left) and the corresponding (CO) intensity map.
<br />Credit: <a href="https://arxiv.org/pdf/1709.09066.pdf" target="_blank" rel="noopener noreferrer">Patrick Breysse from Kovetz et al 2017</a>
</div>
<div class="glos_img" style="background-image:url(images/intensity_map.JPG); "></div>
</li>
<li>
<! INTERFERENCE !>
<div class="glos_def">
<span class="bold">Interference:</span> In physics, interference occurs when two or more waves combine. If the crests and troughs of the waves line up, they combine into a stronger wave, but if the crests of one wave overlap the troughs of another, the wave weakens. With radio telescopes, this interference can be unintentional from a nearby transmitter (like a radio station), which makes astronomical observations extraordinarily difficult!<br />
<span class="img_cred_body">Pages: <a href="spin_flip.html">Spin-Flip Background</a>, <a href="radio_telescopes.html">Radio Telescopes</a>, <a href="lunar_telescopes.html">Lunar Radio Telescopes</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! INTERFEROMETER !>
<div class="glos_def">
<span class="bold">Interferometer:</span> A special kind of observatory - most commonly observing in the radio range - that combines signals from many different individual telescopes into a more powerful observation.<br />
<span class="img_cred_body">Pages: <a href="radio_telescopes.html">Radio Telescopes</a>, <a href="lunar_telescopes.html">Lunar Telescopes</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! INTERGALACTIC !>
<div class="glos_def">
<span class="bold">Intergalactic:</span> Pertaining to or located in the areas between galaxies.<br />
<span class="img_cred_body">Pages: <a href="light_fills_the_universe.html">Light Fills The Universe</a>, <a href="spin_flip.html">Spin Flip Background</a>, <a href="epoch_of_reionization.html">Epoch of Reionization</a>, <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a>, <a href="radio_telescopes.html">Radio Telescopes</a>, <a href="other_telescopes.html">Other Telescopes</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! INTERSTELLAR MEDIUM !>
<div class="glos_def">
<span class="bold">Interstellar Medium:</span> Gas and dust that fills the space between the stars inside of a galaxy. This is the material from which the stars form.<br />
<span class="img_cred_body">Pages: <a href="light_fills_the_universe.html">Light Fills The Universe</a>, <a href="larger_galaxies.html">Larger and Larger Galaxies</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! IONIZE !>
<div class="glos_def">
<span class="bold">Ionize:</span> To change the electric charge of an atom by adding or removing electrons, yielding a net electric (non-neutral) charge. In ionized hydrogen, the proton and electron have been separated, so the spin-flip background cannot be produced.<br />
<span class="img_cred_body">Pages: <a href="spin_flip.html">Spin Flip Background</a>, <a href="epoch_of_reionization.html">Epoch of Reionization</a></span>
</div>
<div class="glos_img_cred">
Image: Energy, such as in the form of a photon, exciting the electron enough so that it breaks it's bond with the hydrogen proton. This leaves a free electron and a positively charged hydrogen ion.
</div>
<div class="glos_img" style="background-image:url(images/ionize.png); background-size:contain;"></div>
</li>
<li>
<! IONOSPHERE !>
<div class="glos_def">
<span class="bold">Ionosphere:</span> The layer of the earth's atmosphere ionized by solar radiation that absorbs and reflects radio waves. The layer is about 50 to 600 miles (80 to 1,000 km) above the surface.<br />
<span class="img_cred_body">Pages: <a href="radio_telescopes.html">Radio Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: A sunset on the Indian Ocean, blue layers are the upper atmosphere including the ionosphere.
<br />Credit: <a href="https://images.nasa.gov/details-iss023e057948" target="_blank" rel="noopener noreferrer">Expedition 23 crew member on the International Space Station (ISS)</a>
</div>
<div class="glos_img" style="background-image:url(images/ionosphere.jpg); "></div>
</li>
<li>
<! IRREGULAR GALAXY !>
<div class="glos_def">
<span class="bold">Irregular Galaxy:</span> A “messy” galaxy with a disturbed configuration, rather than a disk or elliptical galaxy. Usually, such galaxies are either rather small or in the midst of colliding with another nearby galaxy.<br />
<span class="img_cred_body">Pages: <a href="larger_galaxies.html">Larger and Larger Galaxies</a></span>
</div>
<div class="glos_img_cred">
Dwarf Galaxy NGC 1140, a modern galaxy similar to those in the early Universe.
<br />
Image Credit: <a href="https://images.nasa.gov/details-GSFC_20171208_Archive_e000677" target="_blank" rel="noopener noreferrer">NASA Goddard</a>
</div>
<div class="glos_img" style="background-image:url(images/dwarf_first_galaxies.jpg); "></div>
</li>
<li>
<! JAMES WEBB SPACE TELESCOPE !>
<div class="glos_def">
<span class="bold">James Webb Space Telescope:</span> To be launched in late 2021, this infrared NASA telescope will succeed the Hubble Space Telescope and be able to see further into the Universe’s history than any telescope before. It is named after the NASA Administrator James Webb, who ran the program when it was only a ‘fledgling’ space agency in the 1960s, and who many believe led the program to have the high value on science that now motivates missions such as these today.<br />
<span class="img_cred_body">Pages: <a href="larger_galaxies.html">Larger and Larger Galaxies</a>, <a href="space_telescopes.html">Space Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: James Webb Space Telescope concept
<br />Credit: <a href="https://www.jwst.nasa.gov/" target="_blank" rel="noopener noreferrer">Northrop Grumman</a>
</div>
<div class="glos_img" style="background-image:url(images/jwst.jpg); background-size:85%;"></div>
</li>
<li>
<! JETS !>
<div class="glos_def">
<span class="bold">Jets (of a Black Hole):</span> While most of the matter near a black hole flows into it, powerful magnetic fields can deflect some of the material away - and even accelerate it into a long linear jet of material flowing far out of the black hole's environment.<br />
<span class="img_cred_body">Pages: <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a></span>
</div>
<div class="glos_img_cred">
Image: A snapshot of Centaurus A's central black hole's <i>jets</i>.
<br />Credit: <a href="https://images.nasa.gov/details-GSFC_20171208_Archive_e001876" target="_blank" rel="noopener noreferrer">ESO/WFI (visible); MPIfR/ESO/APEX/A.Weiss et al. (microwave); NASA/CXC/CfA/R.Kraft et al. (X-ray)</a>
</div>
<div class="glos_img" style="background-image:url(images/bh_jets.jpg); "></div>
</li>
<li>
<! KECK !>
<div class="glos_def">
<span class="bold">Keck Telescopes:</span> A pair of infrared telescopes on top of Mauna Kea in Hawaii, which are some of the largest telescopes today with primary mirrors about 10 meters across (or over 30 feet).<br />
<span class="img_cred_body">Pages: <a href="infrared_telescopes.html">Infrared Telescopes</a></span>
</div>
<div class="glos_img_cred">
W. M. Keck Observatory in Mauna Kea, Hawaii.gr
<br />
Image Credit: <a href="https://www.nasa.gov/press-release/nasa-awards-new-cooperative-agreement-to-w-m-keck-observatory" target="_blank" rel="noopener noreferrer">Ethan Tweedie Photography/W. M. Keck Observatory</a>
</div>
<div class="glos_img" style="background-image:url(images/keck.jpg); background-size:80%;"></div>
</li>
<li>
<! Light-Year !>
<div class="glos_def">
<span class="bold">Light-Year:</span> The distance light can travel in one year, about six trillion miles (or nearly ten trillion kilometers).<br />
<span class="img_cred_body">Pages: <a href="cosmic_noon.html">Cosmic Noon</a>, <a href="our_galaxy.html">Our Galaxy: The Milky Way</a></span>
</div>
<div class="glos_img_cred">
Credit: <a href="https://exoplanets.nasa.gov/faq/26/what-is-a-light-year/" target="_blank" rel="noopener noreferrer">NASA/JPL-Caltech</a>
</div>
<div class="glos_img" style="background-image:url(images/light_year.jpg); "></div>
</li>
<li>
<! LIGO !>
<div class="glos_def">
<span class="bold">LIGO (Laser Interferometer Gravitational-Wave Observatory):</span> An experiment that detected the first gravitational waves from nearby black holes in 2015. LIGO consists of two widely-separated interferometers within the United States—one in Hanford, Washington and the other in Livingston, Louisiana—operating in unison to detect gravitational waves.<br />
<span class="img_cred_body">Pages: <a href="other_telescopes.html">Other Telescopes</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! LISA !>
<div class="glos_def">
<span class="bold">LISA (Laser Interferometric Gravitational Observatory):</span> Due to launch in the 2030s, this observatory hopes to use gravitational waves to study distant black holes, possibly all the way back to the Cosmic Dawn. <br />
<span class="img_cred_body">Pages: <a href="other_telescopes.html">Other Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: The LISA mission will consist of three spacecraft, using lasers to precisely measure how their separation changes.
<br />Credit: <a href="https://www.elisascience.org/multimedia/image/lisa-mission-proposal-cover" target="_blank" rel="noopener noreferrer">© NASA/Simon Barke</a>
</div>
<div class="glos_img" style="background-image:url(images/lisa.jpg); background-size:85%;"></div>
</li>
<li>
<! LUMINOSITY !>
<div class="glos_def">
<span class="bold">Luminosity:</span> A measure of brightness defined as the total energy emitted by an object per unit time. It’s most commonly measured in joules per second or watts, but astronomers often use other units, especially magnitudes.<br />
<span class="img_cred_body">Pages: <a href="cosmic_web.html">Cosmic Web</a>, <a href="first_stars.html">First Stars</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! LYMAN ALPHA LINE !>
<div class="glos_def">
<span class="bold">Lyman-Alpha Line:</span> An <i>emission line</i> from the hydrogen atom, in which the electron shifts from its first excited state to its lowest-energy state. Because nearly all of the atoms in the Universe are hydrogen, this can be an extremely important line for astronomers.<br />
<span class="img_cred_body">Pages: <a href="infrared_telescopes.html">Infrared Telescopes</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! MESSIER !>
<div class="glos_def">
<span class="bold">Messier:</span> Charles Messier was a French Astronomer in the 18th century who catalogued many of the brightest nebulae in the night sky. His catalog is still used to identify these objects today.<br />
<span class="img_cred_body">Pages: <a href="our_galaxy.html">Our Galaxy</a></span>
</div>
<div class="glos_img_cred">
Image: Messier objects as seen by the Hubble Space Telescope.
<br />Credit: <a href="https://www.nasa.gov/content/goddard/hubble-s-messier-catalog" target="_blank" rel="noopener noreferrer">NASA, ESA/Hubble and the Hubble Heritage Team</a>
</div>
<div class="glos_img" style="background-image:url(images/messier.jpg); "></div>
</li>
<li>
<! METEOR TRAILS !>
<div class="glos_def">
<span class="bold">Meteor Trails:</span> A trail of ionized gas behind a meteor passing through the upper atmosphere that is capable of reflecting radio waves.<br />
<span class="img_cred_body">Pages: <a href="lunar_telescopes.html">Lunar Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: A time-lapse of Perseid meteors, August 2009.
<br />Credit: <a href="https://www.nasa.gov/topics/solarsystem/features/watchtheskies/perseid-meteor-shower-aug11-12.html" target="_blank" rel="noopener noreferrer">NASA/JPL</a>
</div>
<div class="glos_img" style="background-image:url(images/meteor_trails.jpg); "></div>
</li>
<li>
<! MICROWAVE LIGHT !>
<div class="glos_def">
<span class="bold">Microwave Light:</span> A type of electromagnetic wave that is more energetic than radio, but less energetic than infrared. Microwaves have a wavelength from one meter to one millimeter. And yes, microwave ovens use this type of light to heat your food!<br />
<span class="img_cred_body">Pages: <a href="big_bang_cmb.html">Big Bang and CMB</a>, <a href="other_telescopes.html">Other Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: Shows the types of wavelengths going from longest to shortest.
<br />Credit: <a href="https://www.nasa.gov/audience/forstudents/k-4/dictionary/Electromagnetic_Spectrum.html" target="_blank" rel="noopener noreferrer">NASA</a>
</div>
<div class="glos_img" style="background-image:url(images/EM.png); background-size:contain;"></div>
</li>
<li>
<! MILKY WAY !>
<div class="glos_def">
<span class="bold">Milky Way:</span> The galaxy in which our solar system resides. It is a fairly average spiral galaxy today.<br />
<span class="img_cred_body">Pages: <a href="first_stars.html">First Stars</a>, <a href="light_fills_the_universe.html">Light Fills The Universe</a>, <a href="first_galaxies.html">First Galaxies</a>, <a href="larger_galaxies.html">Larger and Larger Galaxies</a>, <a href="radio_telescopes.html">Radio Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: This infrared image from NASA Spitzer Space Telescope shows hundreds of thousands of stars crowded into the swirling core of our spiral Milky Way galaxy.
<br />
Credit: <a href="https://images.nasa.gov/details-PIA03654" target="_blank" rel="noopener noreferrer">NASA/JPL-Caltech</a>
</div>
<div class="glos_img" style="background-image:url(images/milky_way.jpg); "></div>
</li>
<li>
<! MOLECULAR HYDROGEN !>
<div class="glos_def">
<span class="bold">Molecular Hydrogen:</span> A pair of two hydrogen atoms latched together. This configuration is often written as H<sub>2</sub>.<br />
<span class="img_cred_body">Pages: <a href="light_fills_the_universe.html">Light Fills The Universe</a></span>
</div>
<div class="glos_img_cred">
Image: A diagram of two hydrogen bound to form a molecule.
</div>
<div class="glos_img" style="background-image:url(images/molecular_hydrogen.png); background-size:contain;"></div>
</li>
<li>
<! NANCY GRACE ROMAN TELESCOPE !>
<div class="glos_def">
<span class="bold">Nancy Grace Roman Telescope:</span> The Roman Telescope is a future space telescope that will have a wide-angle camera enabling it to survey huge parts of the sky. (In contrast, most astronomical telescopes have tiny fields of view.) It will observe some of the brightest galaxies during the Cosmic Dawn. It is named for Nancy Grace Roman, one of the founders of NASA's astronomy program.<br />
<span class="img_cred_body">Pages: <a href="infrared_telescopes.html">Infrared Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: Nancy Grace Roman Telescope concept
<br />Credit: <a href="https://roman.gsfc.nasa.gov/" target="_blank" rel="noopener noreferrer">GSFC/SVS</a>
</div>
<div class="glos_img" style="background-image:url(images/roman.jpg); "></div>
</li>
<li>
<! NEBULA !>
<div class="glos_def">
<span class="bold">Nebula:</span> A tenuous cloud of gas between the stars. Often, nebulae are the sites of star formation.<br />
<span class="img_cred_body">Pages: <a href="space_telescopes.html">Space Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: Hubble's most famous image: towers of dust and gas in the Eagle Nebula (M16), named 'Pillars of Creation'.
<br />Credit: <a href="https://www.nasa.gov/image-feature/the-pillars-of-creation" target="_blank" rel="noopener noreferrer">NASA, ESA and the Hubble Heritage Team (STScI/AURA)</a>
</div>
<div class="glos_img" style="background-image:url(images/pillars_of_creation.jpg); background-size:60%"></div>
</li>
<li>
<! NEUTRAL HYDROGEN !>
<div class="glos_def">
<span class="bold">Neutral Hydrogen:</span> Atomic hydrogen with no net charge, meaning it contains both a proton and electron.<br />
<span class="img_cred_body">Pages: <a href="big_bang_cmb.html">Big Bang</a>, <a href="cosmic_web.html">Cosmic Web</a>, <a href="exotic_physics.html">Exotic Physics</a></span>
</div>
<div class="glos_img_cred">
Image: Hydrogen, the simpliest atom, in its neutral form.
</div>
<div class="glos_img" style="background-image:url(images/electron.png); background-size:contain;"></div>
</li>
<li>
<! NEUTRON !>
<div class="glos_def">
<span class="bold">Neutron:</span> A subatomic particle with the same mass as a proton but without an electric charge. Neutrons are found in the nuclei of nearly all atoms, except for hydrogen.<br />
<span class="img_cred_body">Pages: <a href="big_bang_cmb.html">Big Bang</a></span>
</div>
<div class="glos_img_cred">
Image: Diagram of helium atom, green indicating neutrons.
</div>
<div class="glos_img" style="background-image:url(images/helium.png); background-size:contain;"></div>
</li>
<li>
<! NUCLEAR FUSION !>
<div class="glos_def">
<span class="bold">Nuclear Fusion:</span> This process occurs when the nuclei of light elements collide and stick together as heavier elements, such as hydrogen combining to form helium. During fusion, some of the matter inside the nuclei of the original elements is converted to energy and released, making the reaction exothermic.<br />
<span class="img_cred_body">Pages: <a href="first_stars.html">First Stars</a></span>
</div>
<div class="glos_img_cred">
Image: A diagram of a particular type of fusion called deuterium–tritium fusion, where deuterium and tritium (forms of hydrogen with neutrons) combine to form helium as well as an extra neutron and release energy. There are many forms of nuclear fusion in a star, combining to make elements up to iron.
</div>
<div class="glos_img" style="background-image:url(images/fusion.png); background-size:contain;"></div>
</li>
<li>
<! PHOTON !>
<div class="glos_def">
<span class="bold">Photon:</span> Particles of light. According to quantum mechanics, light has the properties of both a particle and a wave. But it always travels in discrete packets called photons. <br />
<span class="img_cred_body">Pages: <a href="big_bang_cmb.html">Big Bang</a></span>
</div>
<div class="glos_img_cred">
Image: common representaion for a photon in diagrams.
</div>
<div class="glos_img" style="background-image:url(images/photon.png); "></div>
</li>
<li>
<! POLARIZATION !>
<div class="glos_def">
<span class="bold">Polarization (of Light):</span> Light oscillates as a wave with crests and troughs. Polarization describes the direction in which those oscillations occur. <br />
<span class="img_cred_body">Pages: <a href="epoch_of_reionization.html">Epoch of Reionization</a>, <a href="other_telescopes.html">Other Telescopes</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! POPULATION III !>
<div class="glos_def">
<span class="bold">Population III:</span> See Primordial Stars. <br />
<span class="img_cred_body"></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! PIMARY MIRROR !>
<div class="glos_def">
<span class="bold">Primary Mirror:</span> The largest mirror on a telescope, which determines how much light it collects and how sharply it can image objects. Telescopes have other mirrors (secondaries and tertiaries) to redirect the light reflected by the primary mirror to the cameras or other measuring devices.<br />
<span class="img_cred_body">Pages: <a href="infrared_telescopes.html">Infrared Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: James Webb Space Telescope's <i>primary mirror</i> after the telescope was assembled into its final form.
<br />
Credit: <a href="https://www.jwst.nasa.gov/" target="_blank" rel="noopener noreferrer">NASA/Chris Gunn</a>
</div>
<div class="glos_img" style="background-image:url(images/jim_mirror.jpg); "></div>
</li>
<li>
<! PRIMORDIAL BLACK HOLES !>
<div class="glos_def">
<span class="bold">Primordial Black Holes:</span> A small black hole that formed in the dark ages through direct gravitational collapse or other exotic means, before any stars had first formed.<br />
<span class="img_cred_body">Pages: <a href="exotic_physics.html">Exotic Physics</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! PRIMORDIAL GAS !>
<div class="glos_def">
<span class="bold">Primordial Gas:</span> Gas composed entirely of hydrogen and helium and lacking any heavier elements. This mixture was created just a few minutes after the Big Bang, while heavier elements could not be created until the first stars formed. <br />
<span class="img_cred_body">Pages: <a href="light_fills_the_universe.html">Light Fills The Universe</a>, <a href="spin_flip.html">Spin Flip Background</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! PRIMORDIAL STARS !>
<div class="glos_def">
<span class="bold">Primordial Stars:</span> The first stars in the universe, also known as Population III stars. These stars formed from gas clouds that lacked any elements heavier than hydrogen or helium, so they formed through different mechanisms than stars like our Sun. <br />
<span class="img_cred_body">Pages: <a href="first_stars.html">First Stars</a>, <a href="larger_galaxies.html">Larger and Larger Galaxies</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! PROTON !>
<div class="glos_def">
<span class="bold">Proton:</span> A positively charged subatomic particle found in the nucleus of an atom. The number of protons in the nucleus determines the atomic number of the element.<br />
<span class="img_cred_body">Pages: <a href="big_bang_cmb.html">Big Bang</a>, <a href="cosmic_web.html">Cosmic Web</a>, <a href="spin_flip.html">Spin Flip Background</a>, <a href="epoch_of_reionization.html">Epoch of Reionization</a></span>
</div>
<div class="glos_img_cred">
Image: Hydrogen, the simpliest atom.
</div>
<div class="glos_img" style="background-image:url(images/electron.png); background-size:contain;"></div>
</li>
<li>
<! QUASAR !>
<div class="glos_def">
<span class="bold">Quasar:</span> An extremely bright astronomical object whose exceptional luminosity is generated by gas falling onto a supermassive black hole.<br />
<span class="img_cred_body">Pages: <a href="first_black_holes.html">First Black Holes</a>, <a href="epoch_of_reionization.html">Epoch of Reionization</a>, <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a></span>
</div>
<div class="glos_img_cred">
Image: Artist concept illustrates a <i>quasar</i>, or feeding black hole
<br />Credit: <a href="https://images.nasa.gov/details-PIA16114" target="_blank" rel="noopener noreferrer">JPL</a>
</div>
<div class="glos_img" style="background-image:url(images/quasar.jpg); "></div>
</li>
<li>
<! QUENCHING !>
<div class="glos_def">
<span class="bold">Quenching:</span> Generally meaning to stifle or supress, but in astronomy referred to as the halting of star formation in a galaxy due to a lack of cold gas. Its cause is not entirely clear, but it may be related to powerful quasars inside of massive galaxies.<br />
<span class="img_cred_body">Pages: <a href="first_black_holes.html">First Black Holes</a>, <a href="epoch_of_reionization.html">Epoch of Reionization</a>, <a href="cosmic_noon.html">Galaxies at Cosmic Noon</a></span>
</div>
<div class="glos_img_cred">
</div>
<div class="glos_img" style="background-image:url(); "></div>
</li>
<li>
<! RADIO WAVE !>
<div class="glos_def">
<span class="bold">Radio Waves:</span> Low-energy electromagnetic radiation (or light) with the longest wavelength on the electromagnetic spectrum, as short as 1 millimeter or as long as hundreds of kilometers. <br />
<span class="img_cred_body">Pages: <a href="exotic_physics.html">Exotic Physics</a>, <a href="radio_telescopes.html">Radio Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: Shows the types of wavelengths going from longest to shortest.
<br />Credit: <a href="https://www.nasa.gov/audience/forstudents/k-4/dictionary/Electromagnetic_Spectrum.html" target="_blank" rel="noopener noreferrer">NASA</a>
</div>
<div class="glos_img" style="background-image:url(images/EM.png); background-size:contain;"></div>
</li>
<li>
<! RADIO TELESCOPES !>
<div class="glos_def">
<span class="bold">Radio Telescope:</span> Telescopes that observe light with long wavelengths - or radio waves. Radio telescopes usually act like very powerful satellite dishes. <br />
<span class="img_cred_body">Pages: <a href="radio_telescopes.html">Radio Telescopes</a></span>
</div>
<div class="glos_img_cred">
Image: Parkes telescope in Australia, part of the Commonwealth Scientific and Industrial Research Organization.
<br />Credit: <a href="https://www.jpl.nasa.gov/images/parkes-telescope" target="_blank" rel="noopener noreferrer">CSIRO/Shaun Amy</a>
</div>