Distance Measuring Techniques and The Milky Way Galaxy Measuring distances to stars is one of the biggest challenges in Astronomy. If we had some standard candle, some star with a known luminosity, then we could observe its apparent magnitude, get a distance modulus and determine its distance. It turns out that there is a type of star that can be used as a standard for measuring distance. This star type is called a pulsating variable. 12 13 Such stars are normal stars with varying luminosity. They are not pulsars (which are neutron stars) [and neutron stars are hot neutron spheres, not stars] There are two main types of pulsating variables: RR Lyrae and Cepheids. These stars have varying luminosity because they are not in hydrostatic equilibrium and their size varies periodically. 14 RR Lyrae and Cepheids in the instability strip after leaving the Main Sequence. 15 RR Lyrae variables are normal stars of one solar mass or less in size, averaging ~0.6 M sun. After leaving the Main Sequence, becoming red giants, and starting to burn helium, they go through about a million year period of instability as they settle onto the horizontal branch. They all have pulsation periods of about one-half day to one day. They all have about the same peak luminosity which makes them good prospects for standard candles. 16 All RR Lyrae stars have approximately the same peak luminosity ( ~100 times that of the Sun). Once a star is identified as an RR Lyrae, its absolute magnitude is known, and its distance can be determined [ m M = log(d) 5 + A ] Cepheids have luminosities that are proportional to the length of their periods. Longer periods correlate with higher luminosities. Cepheids are 10 to 100 times brighter than RR Lyrae stars. 17 1
RR Lyrae stars are smaller stars with shorter periods. A Cepheid variable star with a period of about 3 days. A Cepheid showing minimum and maximum brightness in this offset. Cepheids are larger normal stars, from about 3 to 18 solar masses. These stars have evolved off of the Main Sequence and have started burning helium in non-degenerate cores. They are not in hydrostatic equilibrium and are unstable. Their photospheres expand and shrink. Cepheids have a broader range of periods from about one day to over three months. 18 19 By using the correlation between luminosity and period, the period is determined and the absolute magnitude (or luminosity) can be read from the graph. The apparent magnitude is easily measured and, using this with the absolute magnitude ( m - M), the distance to the Cepheid variable can be determined. Relationship between luminosity and period for Cepheids. 22 21 RR Lyrae variables have been an important part of determining distances to other objects in our Milky Way galaxy, and have helped to determine our galaxy s structure. RR Lyrae variable stars are often found in globular clusters and the distances to the globular clusters have been determined using these stars. Cepheid variables are bright enough to see them not only in our galaxy, but in other galaxies, and allow distance measurements as great as ~ 25 Mpc (Million [ Mega ] parsecs). 23 Harlow Shapley used RR Lyrae variables to measure the distance to globular clusters and was the first to understand the true size and shape of the Milky Way halo (announced in 1918). About 150 globular clusters have been found, their positions calculated, and their distribution plotted. They are distributed within roughly a sphere of radius 40 kpc around the center of the Milky Way galaxy. After many years of effort by a large number of dedicated people, a 3-dimensional structure of the entire Milky Way galaxy began to take shape. 24 2
In the early 20th century, there was a big debate about whether or not the Milky Way galaxy constituted the entire universe. Shapley argued that the spiral nebulae were part of the Milky Way. Edwin Hubble, in 1925, discovered Cepheid variables in the Andromeda spiral, M31, and was able to determine its distance as 800 kpc, well outside of the Milky Way galaxy. This proved that M31 was a separate galaxy similar to ours. 25 The great Andromeda galaxy M31 at 800 kpc 26 disk is ~ 500 pc thick 8.5 40 A sample of the distribution of globular Artist s conception of the Milky Way galaxy showing the main populations of stellar objects. clusters in the Milky Way galaxy. 27 28 Stellar Populations Stars can be assigned to different stellar populations, according to the metal abundances (Z values) of elements seen in stellar atmospheres. Population I (young) stars: Z > 0.01 (metal-rich)` Population II (old) stars: Z < 0.001 (metal-deficient) Disk Population (old) stars: intermediate between I and II Populatons, closer to the galactic plane. Recall that Z is the fractional part of the composition for elements heavier than helium. 29 Open cluster relatively new stars, a few hundred to a few thousand, in a loose association, all formed around the same time. Pop I stars. Some of the stars are massive. Globular cluster extremely compact, spherically symmetric, 30 pc diameter balls of up to a million old stars, none of them massive. Pop II stars. All formed around the same time. Disk Population Metallicity increases closer to the disk. Both Pop II and disk population stars are very old. 30 3
Spiral Structure of the Milky Way Galaxy The spiral arms are visible because O and B stars are formed there, along with open clusters. Emission nebulae glow due to the uv radiation from the hot stars. Artist s rendition of the Milky Way galaxy showing the approximate position of our Sun in the disk. The halo is not displayed in this view. 31 Spiral arms are regions of higher density gas and dust, called spiral density waves. The compression of this gas triggers the formation of new stars. 32 NGC 4603 spiral galaxy Distance: 100 million lightyears or ~30 Mpc Similar to the Milky Way galaxy 33 Southern pinwheel galaxy. Spiral galaxy M83 at a distance of ~4.5 Mpc Similar to our own Milky Way galaxy. 34 More on the galactic bulge The bulge is not simply an extension of the disk, but a separate component of the galaxy. The number of stars in the bulge is ~10 billion. The gas density at the center of the bulge is high and there is much star formation activity there. Some heavier elements are detected there which means there have been Type II supernovae that have distributed these elements. 35 More on the galactic halo In the roughly spherical (40 kpc diameter) there are about 150 globular clusters and many single high velocity stars with orbits that are different than the disk. The ages of the globular clusters range from 11 to 13 billion years. There is very little gas and practically no dust in the halo, and few elements heavier than helium. Pop II. There are no young hot stars and no star formation. 36 4
More on the galactic disk The gas and dust in the disk is found primarily in the spiral arms. Most of the massive, hot stars are being formed in the spiral arms. Stars in the disk tend to be Population I stars, which are young and have some heavier elements (i.e., higher metal abundance). 37 Spiral arms must not be tied to the material of the disk because the disk rotates and would wind up the spirals in a few million years and they would disappear. 38 How star formation propagates in the spiral arms. Spiral density waves in the gas and dust of the disk cause new stars to form there. 39 40 North Pole (NGP) star Angle between the galactic equator and the celestial equator is 62.6 o Rotation of galaxy Sun Lat Long center equator around disk Coordinates (Local Standard of Rest) 41 Coordinates (Local Standard of Rest) 42 5
Some facts about the Milky Way galaxy North Pole (NGP) Rotation of galaxy It consists of over 100 billion stars. Sun Long center (0,0,0) Lat star Diameter is ~120,000 light-years ( ~40 kpc). equator around disk Thickness of the spiral disk is about 500 pc. Coordinates (galactocentric system) 43 The central bulge is about 4 kpc thick. 44 Central portion of the Milky Way galaxy The spiral arms contain new stars, open clusters, and much gas and dust. The galaxy is surrounded by a large halo of old individual stars and globular clusters. The halo is about 80 kpc in diameter. At the center, we have the nucleus of the galaxy 45 46 The galactic center 47 48 6
Infrared image Artist s rendition of Sagittarius A at the center of our galaxy, based on actual data. 49 50 Sagittarius A overview Sgr A is a supermassive black hole at the center of the Milky Way galaxy. Mass is about 4 million solar masses. Size is smaller than the Earth s orbit around the Sun (i.e., R s is less than 1 AU). It rotates once every 11 minutes. 51 7