Astronomy 100 - Fall 2003 - Section 1
Lecture 31 (12 November 2003)
Structure of Our Galaxy
Reading: Discovering the Universe IV-1 to IV-3, 14-1 to 14-7
Note to students: these summaries are meant to supplement, not replace, your own in-class notes. I don't guarantee that everything I've said in class will appear here. You are responsible for knowing the material given in the readings and discussed in class.
The Milky Way is a band of apparently diffuse light that stretches across the celestial sphere. Among other constellations, it passes through Sagittarius, where it appears brightest, and Cygnus, where it appears dimmest. On a dark, clear night you can see that it has a patchy structure interrupted by dark clumps.
When a region of this band is viewed through a telescope, however, you can tell that it really consists of the light of a very large number of stars.
In the late 18th and early 19th centuries the astronomers William and Caroline Herschel (brother and sister) mapped the positions of a very large number of stars with their 6-inch reflecting telescope. They concluded that the Milky Way was a flattened disk of stars about 6400 ly across and 1300 ly thick, with the Sun near the center.
The reason for this conclusion is that in looking toward the Milky Way you are looking along the disk, so your line of sight intercepts more stars than if you are looking 90 degrees away from the Milky Way.
Spiral nebulae
The so-called spiral nebulae were catalogued by Charles Messier and Herschel in the 17th century. These “nebulae” appeared as diffuse grey blobs on the sky – Messier in particular catalogued them as “objects that are not comets,” since he was more interested in discovering new comets. (Research priorities change with time!)
The spiral structure in these “nebulae” did not become visible until the construction of the 1.8-m Leviathan telescope in Ireland around 1840. The builder of this telescope, an amateur aristocrat named William Parsons, Third Earl of Rosse, used it to map out spiral patterns in several of the brighter spiral nebulae, such as M51.
Two basic views developed regarding these spiral nebulae and the nature of the Milky Way:
Herschel's view, that the Milky Way is a flattened disk of stars and encompasses all there is, and that the spiral nebulae are gas clouds inside or near the disk of stars.
The view of Immanuel Kant, who argued that the spiral nebulae were also flattened disks of stars (“island universes”), equal to the Milky Way.
The Herschelian picture was the consensus view at the turn of the 20th century. This was typified by the massive star-counting project undertaken by Jacobus Kapteyn around 1900.
Establishing the correct distance scale for galaxies
Two important twentieth-century developments allowed the correct distance scale for and nature of spiral nebulae to be determined.
These are pulsating supergiant stars whose surfaces expand and contract regularly because they are not in hydrostatic equilibrium. When a Cepheid is smaller than its average size, outward radiation pressure is greater than the inward force of gravity, so the star expands and cools. As it expands, the outer layers decrease in density and become more transparent, allowing radiation to escape and decreasing the effectiveness of radiation pressure. So gravity exceeds outward pressure and causes the star to contract again.
The pulsation of a Cepheid causes its brightness to fluctuate in a measurable way. We can measure the apparent brightness of the star and the amount of time between successive peaks in brightness (the period).
In 1912 Henrietta Leavitt discovered that the period of a Cepheid variable star depends on its average luminosity. The longer the period is, the brighter the star is. Leavitt was able to calibrate this relationship by observing Cepheids in the Magellanic Clouds and using other distance estimators for the Clouds to figure out the Cepheids' luminosities.
By measuring the period and apparent brightness of a Cepheid, you can infer its luminosity and hence its distance. Also, since Cepheids are very bright, they can be seen a long way away, so they can be used to measure the distances to nearby galaxies.
It actually turns out that there are two kinds of Cepheids. We will come back to this when we discuss the discovery of the expansion of the Universe.
Discovery of interstellar dust absorption
Until the 20th century it was believed that the spaces between the stars were completely devoid of matter.
In 1930 R. J. Trumpler discovered that significant quantities of dust exist in interstellar space, and that this dust makes stars look dimmer by preferentially scattering optical light.
Because of dust, we see fewer stars than we might otherwise in the direction of the center of the Milky Way. In fact, if we consider that none of the stars on the other side of the center of the Galaxy can be seen in visible light, we see how astronomers might have thought the Sun was close to the center of the Milky Way.
Harlow Shapley used the Cepheid variable technique pioneered by Leavitt to measure the distances to a number of globular clusters around 1920.
Knowing their distances from us and positions in the sky, he could deduce their three-dimensional distribution in space.
The center of this distribution was located 26,000 ly away in the direction of Sagittarius. (This is the correct value, but not the value Shapley obtained – he thought the distance was quite a bit larger.)
Arguing that the center of the distribution must also be the orbital center of the globular clusters, and hence the center of the Milky Way Galaxy, Shapley concluded that the Sun is not located near the center of the Galaxy.
Shapley debated Heber Curtis regarding the nature of the spiral nebulae and the size of the Galaxy in a famous debate held in 1920.
Shapley argued that
the Galaxy is all there is (wrong)
we are not near its center (right)
the spiral nebulae are smaller than the Galaxy and relatively nearby (wrong)
Curtis argued that
we are near the center of the Galaxy (wrong)
spiral nebulae are galaxies (right)
spiral nebulae are large and distant (right)
Each man got some things right and others wrong. The basic issues were eventually settled by Hubble's discovery of Cepheids in the Andromeda Galaxy (1920s) and the discovery of interstellar dust absorption (1930s).
A second Great Debate was held on the 75th anniversary of the first one, “pitting” Don Lamb “against” Bohdan Pacynski on the topic of gamma-ray bursts.
We now know that the spiral nebulae are, in fact, galaxies like our own, very distant from us, and that we are not at the center of our own Galaxy.
The structure of the Milky Way
21-cm emission from atomic hydrogen
Since the 1950s we have been able to map the structure of the Galaxy's disk using 21-cm radio emission from neutral hydrogen clouds in the disk. This radiation is not strongly absorbed except in the center of the Galaxy, so we can see a much larger fraction of the whole than we can using the visible light from stars.
The different parts of the disk viewed along a line of sight have different line-of-sight speeds, so the 21-cm emission produced in different parts of the disk has different Doppler shifts.
Using these observations we've been able to determine that the Milky Way has the following “parts.” (The parts are not solid, of course, but consist of stars and gas clouds.)
A flattened disk about 30 kpc across and less than 1 kpc thick. The disk has several spiral arms like the ones seen in other galaxies.
A smaller bulge a few kpc across at the center of the Galaxy. The stellar population of the bulge is more similar to that of the globular clusters than the disk.
A large, spherical halo more like 100 kpc across. The density of stars in the halo is much lower than in the disk or bulge, but (see below) there's still a lot of matter there.
We are inside the disk about 8 kpc from the center.
The disk rotates differentially: the linear velocity of atomic hydrogen clouds is roughly the same (about 220 km/s) throughout the disk, which means that the angular speed varies with distance from the center. At the Sun's position we go around once in about 230 million years.
Dark matter
The constant rotation velocity is a puzzle (first pointed out by Vera Rubin). The number of stars decreases as we go from the center to the edge of the disk, so if stars (and gas clouds) were all of the matter, the rotation velocity should decrease toward the edge. This is because the orbital velocity depends on the amount of matter within the orbit.
Though there are about 200 billion stars in the Milky Way, the total mass inferred from the rotation velocity is about 1 trillion times the mass of the Sun. So most of the mass is not visible: we think it must be some type of dark matter. (An alternative idea is that Newton – and Einstein – are wrong about gravity; but so far this has not borne out.)
Two basic categories of idea about dark matter:
Perhaps it's in the form of planets, brown dwarfs, black holes, etc. that are so dim and far away that we can't see them. These are called MACHOs (Massive Compact Halo Objects).
Perhaps it's in the form of an exotic type of particle that only interacts with ordinary matter through the force of gravity – a WIMP (Weakly Interacting Massive Particle).
MACHOs can be detected through a technique called gravitational microlensing.
When a MACHO passes in front of a more distant star, light from the star is bent slightly by the MACHO's gravitational field, and we see the star appear to brighten as a result. As the MACHO moves in front of the star, the star gets first brighter, then dimmer.
So by staring at a region of sky every night for a few years and tracking the apparent brightness of all of the stars in that region (with a computer!), we can see how many appear to temporarily brighten in this way. (In any field of view there will always be a number of intrinsically variable stars [like Cepheids] – there are ways to distinguish microlensing events from these.)
From the amount and duration of brightening we can determine the masses and velocities of the MACHOs, and by counting these up we can estimate the total amount of mass in MACHOs.
It turns out that MACHOs can only be a small fraction of the required dark matter.
So – we are left with WIMPs. But nobody has ever seen these: what might they be? Stay tuned.
The Milky Way has several smaller satellite galaxies, including the Sagittarius dwarf elliptical and the Large and Small Magellanic Clouds. (The Clouds are readily visible to the naked eye – if you're in the southern hemisphere.)
The Milky Way and the Andromeda Galaxy, about 0.7 Mpc away, are the two largest members of a small group of galaxies called the Local Group, which contains about 40 galaxies.
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