The Milky Way and Beyond (41 page)

Scorpius X-1 was observed in visible light for the first time in 1966. Optically it is much less impressive, bluish in colour and appearing only faintly. Scorpius X-1 is a close double star, one component of which is optically invisible—a neutron star. The X-rays are generated when matter from the optically visible, bluish hot star falls onto the neutron star. This matter is tremendously accelerated and crushed by the enormous gravity of the neutron star. Unlike the majority of binary X-ray sources, the visible member does not appear to be very massive; it is only 42 percent the mass of the Sun. The neutron star is 1.4 solar masses. Scorpius X-1 is about 9,000 light-years from Earth.

S Doradus is a variable supergiant star in the Large Magellanic Cloud. S Doradus (and the Large Magellanic Cloud) is visible to viewers in the Southern Hemisphere in the constellation Dorado. It is one of the most luminous stars known, radiating more than 1,000,000 times as much energy as the Sun.

Spica (Latin: “Head of Grain”) is the brightest star in the zodiacal constellation Virgo and one of the 15 brightest in the entire sky, having an apparent visual magnitude of 0.98. It is a bluish star; spectroscopic examination reveals Spica to be a binary with a four-day period, its two components being of the first and third magnitudes, respectively. Spica lies about 250 light-years from Earth.

Supernova 1987A was the first supernova observed in 1987 (hence its designation) and the nearest to Earth in more than three centuries. It occurred in the Large Magellanic Cloud, a satellite galaxy of the Milky Way Galaxy that lies about 160,000 light-years distant. The supernova originated in the collapse and subsequent explosion of a supergiant star, and it is unique in that its progenitor star had been observed and cataloged prior to the event. The fact that the supergiant was hotter than expected for an immediate progenitor led to important improvements in supernova theory. A burst of neutrinos that accompanied the star's collapse was detected on Earth, providing verification of theoretical predictions of nuclear processes that occur during supernovae. Study of the evolving remnant continued into the 21st century.

Tycho's Nova (SN 1572) was one of the few recorded supernovae in the Milky Way Galaxy. The Danish astronomer Tycho Brahe first observed the “new star” on Nov. 11, 1572. Other European observers claimed to have noticed it as early as the preceding August, but Tycho's precise measurements showed that it was not some relatively nearby phenomenon, such as a comet, but at the distance of the stars, and that therefore real changes could occur among them.

The supernova remained visible to the unaided eye until March 1574. It attained the apparent magnitude of Venus (about -4) and could be seen by day. There is no known stellar remnant but only traces of glowing nebulosity. It is, however, a radio and X-ray source. In 2008 a team of international astronomers used light from the original explosive event reflected off nearby interstellar dust to determine that Tycho's Nova was a Type Ia supernova, which occurs when a white dwarf star accretes material from a companion star and that material explodes in a thermonuclear reaction that destroys the white dwarf.

binary star
A pair of stars in orbit around a common centre of gravity.

black hole
An area in space with an intense gravitational field whose escape velocity exceeds the speed of light.

cloud cores
The densest regions of molecular clouds, where stars typically form.

dwarf stars
Low-luminosity stars.

eclipsing binary
Two close stars moving in an orbit so placed in space in relation to Earth that the light of one can at times be hidden behind the other.

extrasolar
Revolving around stars other than the Sun.

interferometer
An instrument that combines light waves from two or more different optical paths and can be used to measure the angle subtended by the diameter of a star at the observer's position.

kinematics
The study of motion.

light-year
The distance that light waves travel in one Earth year.

nebulae
A mass of interstellar gas and dust.

nova
A star that brightens temporarily while ejecting a shell explosively.

photons
Packets of radiation.

protostars
A contracting mass of gas that represents an early stage in the formation of a star, before nucleosynthesis has begun.

pulsars
Neutron stars that emit pulses of radiation once per rotation.

quasars
The abbreviation for quasi-stellar radio sources, which emit up to 100 times as much radiation as an entire galaxy.

recombination
The process by which the higher stage of ionization captures an electron, usually at low energies, into a high level of the ion.

thermal ionization
The process at higher temperatures where collisions between atoms and electrons and the absorption of radiation tend to detach electrons and produce singly ionized atoms.

z distances
Distances above the plane of the Galaxy.

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Shrouds of the Night: Masks of the Milky Way and Our Awesome New View of Galaxies
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The de Vaucouleurs Atlas of Galaxies
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Galaxy: Exploring the Milky Way
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Nebulae and How to Observe Them
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The Black Hole at the Center of the Milky Way
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Universe: Stars and Galaxies
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Stellar Spectral Classification
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An Introduction to the Sun and Stars
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The Origin and Evolution of Planetary Nebulae
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A

Albireo,
54

Aldebaran,
46
,
68

Algol,
61
,
112

Alpha Centauri,
13
,
46
,
49
,
50
,
53
,
122

Altair,
46
,
123

Ambartsumian, Victor A.,
117
–118

Andromeda Galaxy,
18
,
28
,
29
,
37
,
121
,
146
,
180
,
184
,
186
,
187
,
190
,
195
–196,
197
,
198

Antares,
46
,
79
,
89

Arcturus,
46
,
68
,
122
,
123

astronomical unit, definition of,
60

B

Baade, Walter,
28
–29,
151

Barnard's star,
39
,
50

Bell, Jocelyn,
106

Bessel, Friedrich W.,
61
,
126

Beta Canis Majoris,
73
,
74
,
75

Beta Pictoris,
67

Betelgeuse,
46
,
54
,
75
,
79
,
123

big bang,
97
,
98
,
109
,
148
,
151
,
165
,
167

binary/double stars,
46
,
50
–51,
52
,
56
,
63
–67,
69
,
70
,
71
,
76
–77,
78
,
79
,
80
,
82
,
93
,
96
,
101
,
102
,
104
,
106
,
107
,
108
,
109
,
112
,
118
,
121
,
125
,
126
,
157
,
169

eclipsing binary stars,
60
,
61
–62,
67
–68,
72
,
73
,
77
,
80
,
82
,
112
,
115

spectroscopic binary stars,
60
,
61
,
62
,
63
,
67
,
124

visual binary stars,
60
–61,
67
,
72

black dwarfs,
104

black holes,
14
,
22
,
77
–78,
79
,
100
–101,
108
–109,
156
,
192
,
193
,
194
,
200

blue stragglers,
116

bolometric magnitude,
55

Book of the Fixed Stars
,
195

Bright Star Catalogue
,
57

brown dwarfs,
27
,
35
,
45
,
52
,
57
,
59
,
60
,
65
,
94
–96,
186

C

Canopus,
46
,
53
,
122

Capella,
46
,
67
,
68
,
88

carbon cycle,
86
–87,
88
,
92

Cassiopeia A,
156
,
159
–160

Cassiopeia-Taurus Group,
36

Cepheid period-luminosity (P-L) law,
184

Cepheids,
29
,
38
,
72
,
73
,
74
,
75
,
76
,
78
,
112
,
115
,
121
,
124
,
168
,
170
,
173
,
174
,
175
,
176
,
183
,
184

Chandrasekhar limit,
101
,
103

charge-coupled devices (CCDs),
132

Clark, Alvan,
126

cloud cores,
90
–91

Coalsack,
138
,
160

Coma Berenices,
110
–111,
114
,
196
–197

Coma cluster,
196
–197

Cosmic Background Explorer,
44

cosmic rays,
99
,
102
,
139
,
155

Crab Nebula,
79
,
99
,
107
,
157
–158

Crab Pulsar,
106
,
107

Cygnus A,
197

Cygnus Loop (Veil Nebula),
158

Cysat, Johann,
162

D

dark energy,
78
,
101

dark matter,
27
–28,
127
,
186

Darquier, Augustin,
162

Delta Scuti,
73
,
76

Deneb,
54
,
123

density distribution of stars,
35
–38

density-wave pattern/theory,
24
,
37

diffuse ionized gas,
129
,
130
,
158
–159

Draper, Henry,
162

E

echelle spectrograph,
133

eclipsing binary stars,
60
,
61
–62,
67
–68,
72
,
73
,
77
,
80
,
82
,
112
,
115