In studying the Milky Way, we have a classic problem of not being able to see the forest for the trees. A panoramic painting of the Milky Way as seen from Earth, done by Knut Lundmark in the 1940 s.
The first sketch of what our Galaxy looks like was made by Sir William Herschel, based upon simply counting stars in various directions.
What Herschel didn t know about was gas and dust.
In 1917, Harlow Shapley estimated our location in the Galaxy using Globular Clusters Globulars are old star clusters, distributed in more-or-less random directions around the center of the Galaxy, but clustered toward the center. Shapley noticed that they do not appear uniformly spread across the sky, and so we must be well away from the center of the Galaxy (he estimated about 30,000 ly--today we would say about 26,000 ly).
To measure the distance, he needed a Standard Candle A standard candle is something for which you know the actual brightness, and it is the same from one object to the next. For example, if you had a light meter, you could determine how far away a 100 W bulb was, by measuring its apparent brightness, and comparing to the brightness of another 100 W bulb that was a known distance from you.
The Standard Candle for globular clusters was RR Lyrae stars RR Lyrae stars are variable stars Due to an internal instability, they pulsate slowly, which we see as a variation in their brightness. The period of the pulsation depends upon the average brightness of the star, in a way that we know. We can examine RR Lyraes in nearby clusters, whose distances we can measure by other means. This allows us to calibrate the period-brightness relation for RR Lyrae stars. We can then measure the period of RR Lyraes in more distant star clusters We can also measure their apparent brightness. The period-luminosity relation tells us how bright the star really is. We therefore know how far away the star must be in order to appear as bright as it does.
Question: You measure the apparent brightness of two stars, let s call them Stan and Oliver. You know that the two are identical, but Oliver appears to be 16 times fainter than Stan. How do their distances from you compare? a) Oliver is 16 times farther away than Stan. b) Oliver is 16 times closer than Stan. c) Oliver is 4 times farther away than Stan. d) Oliver is 2 times farther away than Stan.
We can get an idea of what the Milky Way looks like by analogy with other galaxies, but not its exact structure.
We can map out the shape of the Milky using several means. Looking at hot, young stars tells us the local structure of the Galaxy. Nearby stars are not uniformly distributed, but follow local spiral arm structure. Unfortunately, dust and gas block our visible light view more than a few thousand light years away.
We must use longer wavelengths in order to look at a larger fraction of the Milky Way Longer wavelengths of electromagnetic radiation penetrate better through dust. Shorter wavelengths tend to be scattered.
Visible light Infrared
An infrared image of the Milky Way from the Cosmic Background Explorer Satellite. IR penetrates the gas and dust clouds, giving us a better picture of what the Milky Way looks like.
Question: Why is the sky blue?
To look further out, we must use radio waves. Molecules in interstellar clouds emit radio waves. 21 cm radiation comes from hydrogen atoms. Protons and electrons possess spin, ie. they act as if they were spinning like tops. The spin gives them a magnetic moment, which means that they act a bit like bar magnets. If the spin of the proton and electron are parallel or antiparallel, the atom has a slightly different energy, and a 21 cm photon is either absorbed or emitted when the transition occurs. 21 cm photon emitted Proton and electron spins parallel Proton and electron spins antiparallel
How does it work? 1. Assume that all orbits are circular (not bad) 2. Assume a relation between orbital speed and distance 3. For each circle, there is one unambiguous point (the tangent) 4. For all others, there are two possible distances for a cloud with a given observed velocity. Can use average size to try and sort these out. 5. If it doesn t look good, double-check what you used in step 2, alter it, and see if you can clean up the fit.
A 21 cm radio map of the Milky Way Galaxy.
We now have a reasonable idea of what our Galaxy looks like. An artist s conception (with some artistic license taken).
Question: In 21 cm radio observations in a particular direction, you see blueshifted peaks at several wavelengths from different clouds. The peak that is the most blueshifted probably corresponds to a cloud that is: a) Closer to the Galactic center than the other clouds. b) Farther away from the Galactic center than the other clouds. c) Moving directly away from the Galactic center. d) Moving directly across our line of sight.
Viewed edge-on from outside, the Milky Way would look something like this.
The Milky Way is a galaxy of the type known as spirals. Galaxies similar to the Milky Way......viewed face-on......and edge-on.
Components of the Milky Way Disk: Includes the spiral arms. Contains youngest stars, gas and dust clouds. Follows circular orbits around the Galactic center. Dynamically cold (small random motions-- very ordered). Bulge: Dense stellar region near the center of the Galaxy. Mostly old stars. Orbiting, but dynamically hotter than the disk. Halo: Very old stars, essentially no gas or dust. Following almost random orbits around the Galaxy. Least massive component (< 10% of the disk mass). Disk Bulge Halo
What are the spiral arms? They are not dense regions of stars moving together. The Galaxy rotates differentially, with stars near the center orbiting in less time than stars further out. Differential rotation would wrap up such arms very quickly.
The arms are actually density waves, through which stars and gas pass as they orbit around the Galaxy. When material encounters the wave, it piles up, forming a density enhancement. Shocks and collisions between clouds triggers star formation, and so youngest stars are found just downstream of the density wave.
Question: Which of the following would you expect to best trace out the spiral structure of the Milky Way? a) Hot young stars and interstellar gas. b) Old, evolved stars such as RR Lyrae variables. c) Globular star clusters. d) Old stars moving with large random velocities.
Do we see everything that is there in our Galaxy? The orbital speed of stars around the Galaxy depend upon how much mass is inside of the orbit. If the Milky Way were as concentrated as it looks (from the stars and gas that we see), then stars further out than the Sun should orbit more slowly around the center of the Galaxy than the Sun.
They don t. Something else is out there. But what? Black holes? Brown dwarfs? Jupiters? Something more exotic?
Now, let s take a ride to the center Question: what wavelength are we looking at in the final image? a) Ultraviolet b) Visible c) Infrared
The center of the Milky Way is very different from out here in the suburbs It isn t all old stars. Some very young, dense star clusters are being formed within 100 ly of the Galactic center. They are shown below in images taken by the Space Telescope. 2 Myr old 4 Myr old Infrared images
How do massive clusters of young, high mass stars get there? The clusters cannot survive for long. The tidal fields so close to the Galactic center will tear the clusters apart within a few million years (even as dense as they are). They could not have formed further out, and somehow wandered in. They contain many massive stars. Stars so massive do not live for more than about 5 Myr. They must, therefore, have formed near their current locations, but how?
We may have to modify our picture of star formation Most star formation occurs in dense, very cold gas clouds. These are called molecular clouds, because most of the gas within them consists of molecules, rather than individual atoms. They also contain much dust. A trigger, either cooling or compression, gives gravity the upper hand within part of the cloud, and it contracts and breaks up to form stars. The Galactic center, however, is a hostile environment for star formation. Gas temperatures are high. Pressures are high. There is much high energy radiation.
The center of the Galaxy is an extremely active place The central region of the Galaxy has an average density of matter about a million times greater than near the Sun. Clusters of massive stars are being formed, supernovae occur frequently, and there are very complex gas motions.
At the very center, we see gas which is probably orbiting around a massive black hole. A bright radio object, called Sagittarius A (or just Sgr A) lies at what we believe to be the center of the Galaxy. It is shown below as seen in radio waves. The orbital speeds of the gas indicate that it is in orbit around an object with a mass of about 10 6 times that of the Sun!
Stellar motions also indicate something lurking in the center Movie made by Andrea Ghez (UCLA), from her observations of stars in the central parsec (about 3.26 ly) of the Galaxy
Andrea Ghez has also observed brightness variations of the object thought to be the actual black hole. These are most likely due to matter falling into the black hole itself. The bar is about 0.004 pc, or 20 times the size of Pluto s orbit!
The black hole in the Milky Way is almost certainly a cousin of even bigger monsters which we believe to exist in other galaxies.