Download The Milky Way Galaxy

The Milky Way Galaxy
Robert S. French, HET603/Baade, Swinburne Astronomy Online
Word count (excluding title and references): 1,923
All of the work contained in this essay is my own original work, unless otherwise clearly stated and
referenced.
Introduction
Of the billions of galaxies in the universe, the Milky Way is the only one that we can examine in detail.
Only in the past hundred years has the structure of the Milky Way begun to reveal itself with the advent
of new observational technologies such as radio, infrared, ultraviolet, and X-ray telescopes.
The Milky Way is believed to be a barred spiral galaxy about 100,000 light-years in diameter. There is
a visible disk 1,000 light-years thick, a massive central bulge about 3,000 light-years in diameter, and a
mostly spherical halo surrounding the galaxy containing globular clusters, old stars, and dark matter.
Our solar system exists about 26,000 light-years from the center at the edge of one of the spiral arms
(Belkora 2003). While we may think of the Milky Way as a self-contained object, recent research
shows that it has merged with, and is continuing to merge with, other small, nearby galaxies. We will
explore the structure of the Milky Way and discuss the recent findings about the cannibalism of these
neighboring dwarf galaxies.
The Visible Disk and Arms
The visible disk is a large, circular collection of stars, gas, and dust aligned with the galactic midplane.
It is about 1,000 light-years thick and extends 50,000 light-years from the center (Belkora 2003). The
stars, gas, and dust generally all orbit in the same direction around the center of the galaxy at
approximately the same speed (200 km/sec) regardless of their distance. Analysis of this rotation allows
astronomers to estimate the amount and distribution of mass in the galaxy. Only about 10-20% of the
predicted mass is accounted for through observation and the rest is assumed to be an as-yetundiscovered form of matter called dark matter (Pasachoff & Filippenko 2007), much of which should
be in the halo (Belkora 2003).
As a spiral galaxy, the Milky Way has multiple arms. The exact layout of the arms is unknown due to
the difficulty in seeing through the dust in the disk. However, recent observations in the infrared and
sub-millimeter wavelengths have given us our first glimpse of their structure (Pasachoff & Filippenko
2007). See Figure 1 for an artist’s rendition based on current knowledge.
The spiral structure of the galaxy and the movement of the arms is not indicative of star motion, but
instead indicates the movement of density waves. These density waves cause regions of higher density
to move around the galaxy in much the same way that a traffic jam moves slowly down the highway, but
individual cars move much faster (Sugiyama et al. 2008). The density waves move at a slower rate than
the stars, and thus the stars orbit around the center of the galaxy more quickly than the arms do. These
density waves cause clouds of gas to compress, which creates regions of star formation.
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Figure 1: An artist’s rendition of the Milky Way including the central bar and arms (AtlasWeb).
At visible wavelengths, the arms look bright with little luminosity between them. However, this is not
representative of the number of stars. The arms, being the site of accelerated star formation, generally
contain hot, young stars that are very luminous, while older, cooler stars are found between the arms.
The density of stars in the arms is only 10% greater than that between the arms, but these stars account
for the majority of visible light (Belkora 2003).
The Sun moves with a speed of about 68 km/sec relative to the arms, passing through a spiral arm every
200 million years or so. It is currently headed towards the Perseus arm, which it will enter in another
140 million years (Belkora 2003). Leitch & Vasisht (1998) demonstrated that the three most recent arm
crossings correlated with mass extinctions on Earth. They theorized that since arms are regions of heavy
stellar formation activity and instability that this would have increased the probability of an impact on
Earth, or a supernova in the immediate vicinity, which would have caused the extinction.
The Interstellar Medium
The area between the stars, called the interstellar medium, is a hard vacuum with an average density of
only 1 atom/cm3. It is generally filled with neutral cold hydrogen, although the distribution is not
homogeneous. For example, giant molecular clouds can contain the mass of more than a million Suns
and span 150 light-years. The density at the core of a giant molecular cloud can exceed 107 atoms/cm3,
and this is where stars are born (Pasachoff & Filippenko 2007).
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Other regions contain hydrogen ionized by the formation of hot, young stars and heated to temperatures
of 10,000° K that cause them to emit light. These regions are called emission nebulae (UCSDWeb).
One of the most spectacular and well-known emission nebulae is the Orion Nebula, which is 1,300 lightyears from the Sun. It is about 20,000 times the diameter of the solar system and contains enough gas to
form tens of thousands of stars (Belkora 2003).
Finally, about 1% of the interstellar medium is dust composed of carbon and silicates. This dust is the
primary reason that we are unable to see very far through the galactic disk because it absorbs and
scatters visible light (UCSDWeb).
The solar system exists on the Orion spur between the Sagittarius and Perseus arms in an area called the
local bubble. The local bubble is much less dense that the average interstellar medium, with about 0.1
atoms/cm3. The gas is very hot (around 106° K), causing it to emit X-rays. The local bubble has a
radius of about 300 light-years, but is elongated perpendicular to the Galactic plane (Breitschwerdt
2001). The origin of the local bubble is still under debate. Many astronomers believe that it was carved
out by a series of supernovae. Berghöfer & Breitschwerdt (2002) estimate that 20 supernovae must have
exploded in the area within the past 10-20 million years, which is the approximate age of the local
bubble, and that the stars for these supernovae were part of the Pleiades moving group (Asiain et al.
1999). The most recent supernova occurred about 1 million years ago, reheating the gas in the local
bubble.
The Central Bulge
In the center of the visible disk is the central bulge, which is about 3,000 light-years in diameter. It
extends 1,800 light-years above and below the visible disk, and contains the mass of 10 billion Suns.
The center of the bulge contains a bar that is about twice as long as it is wide, with the long axis pointing
about 45 degrees to the Sun (Belkora 2003). See Figure 2 for a photograph of the bulge, and Figure 1
for an artist’s depiction of the bar.
Deep inside the central bulge is the nucleus, which is only about 10 light-years in diameter (Pasachoff &
Filippenko 2007). It contains a vast rotating ring of dust and gas, the circumnuclear disk, surrounding a
central cavity. The cavity contains hundreds of thousands of stars, including red giants and massive blue
stars. At the center of the central cavity is a dense collection of stars and a mysterious strong, compact
radio source called Sagittarius A* (Morris & Serabyn 1996).
Only recently has sub-millimeter and infrared imaging allowed us to see the center of the galaxy
directly. Strong evidence points to the existence of a supermassive black hole with a mass of 3.7x106
Suns that is very close to Sagittarius A* (Ghez et al. 2005). Orbiting the black hole are approximately
40 young luminous stars with masses of 30-120 Suns and ages of 2-7 million years. One recently
identified star passed within 45 AU of the galactic center at a speed of 12,000 km/sec, 250 times faster
than Mercury orbits the Sun. The origin of these young, massive stars is unknown, and it is troubling
that they could be formed so close to a supermassive black hole because tidal forces should prevent their
creation. Their existence is one of the newest and most exciting issues in the study of the galactic center
(Lu et al. 2005).
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Figure 2: A view of the Milky Way taken in infrared light by the COBE satellite. The central bulge is
clearly visible (UCLAWeb).
The Stellar Halo
Surrounding the visible disk is a large, mostly spherical region called the stellar halo. This area is
populated by approximately 150 globular clusters, each of which contains hundreds of thousands or
millions of very old metal-poor stars in an area only a few light-years across. The globular clusters
move along randomly oriented orbits and spend most of their time in the halo (Sparke & Gallagher
2007). An outer halo, consisting primarily of dark matter, may extend 200,000 light-years from the
center (Pasachoff & Filippenko 2007), and is required to explain the orbits of stars in the galaxy.
Cannibalism of Dwarf Galaxies
One of the most intriguing discoveries over the past 25 years has been evidence that the Milky Way has
“eaten,” or is still in the process of eating, other, smaller galaxies. These discoveries are made by
observing large collections of stars and noting cases where subgroups have a similar velocity that is
unrelated to the general orbit of stars around the center of the galaxy.
For example, Gilmore et al. (2002) showed that a large collection of stars near the galactic plane
probably came from a satellite galaxy that merged with the Milky Way 10-12 billion years ago. They
also showed that the halo contains evidence of various minor mergers as well, although (JHUWeb)
posited that at most 10% of the halo’s stars could have come from mergers with other galaxies.
Likewise, Grillmair (2006) detected a long, thin stream of stars extending from Ursa Major to Sextans at
a distance of 70 light-years. The stream is most likely the remains of another dwarf galaxy that merged
sometime in the distant past.
The most famous case, though, is the gradual merging of the Sagittarius dwarf galaxy with the Milky
Way. The Sagittarius dwarf galaxy has about 1/10th the diameter and 1/1000th the mass of the Milky
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Way. One of about a dozen known companion dwarf galaxies, it orbits in less than 1 billion years, and
has orbited at least ten times since the formation of the Milky Way. It plunges deep inside the Milky
Way on each orbit. The dwarf galaxy should have been completely pulled apart by tidal forces from the
Milky Way by now, but hasn’t been, which means it must be much more massive than we think. This
has been taken as additional evidence for the existence of dark matter (JHUWeb).
During each orbit stars are tidally stripped from the passing galaxy and appear as groups of stars in the
halo with similar velocity vectors (Johnston et al. 1995). This stream of stars, called the Sagittarius
stream, orbits within 13 degrees of a polar orbit (Ibata et al. 2001) and passes through the galactic disk.
Belokurov et al. (2006) showed that the motion of the stream suggests that the halo is more or less
spherical. The Sagittarius stream goes through the galactic disk but passes about 48,000 light years from
the Sun. In fact, Seabroke et al. (2008) found no evidence that any stream passes through the solar
neighborhood.
Conclusion
We have discussed the structure of the Milky Way galaxy, including the visible disk, interstellar
medium, central bulge, nucleus, and stellar halo. Recent evidence also confirms that the Milky Way has
merged with several smaller dwarf galaxies, and is in the process of merging with the Sagittarius dwarf
galaxy at this time, although none of these mergers affect the solar neighborhood.
References
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AtlasWeb: http://www.atlasoftheuniverse.com/milkyway.html
Belkora, L. 2003, Minding the Heavens: The Story of Our Discovery of the Milky Way (Bristol:
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Belokurov, V., et al. 2006, ApJ, 642, L137
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UCLAWeb: http://www.astro.ucla.edu/%7ewright/DIRBE/dirbe123_2p6dec.jpg
UCSDWeb: http://cass.ucsd.edu/public/tutorial/ISM.html
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