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Home , Harlow Shapley

Our Learning Objectives

! What is the Milky Way? The Herschels thought we were at the center of our ...why were they wrong? ! How did Shapley prove we aren’t at the center? What are globular clusters? stars? ! How do we use Cepheid Variables to measure distance? ! What are the components of our Galaxy? What color are old stars? Young stars? Does our Galaxy get older or younger as you move out (i.e. from the disk to the halo)? ! How do we know our Galaxy is a ? ! Do stars in our Galaxy’s disk orbit as Kepler’s Laws would predict? What is a rotation curve? Why does our Galaxy’s rotation curve suggest dark matter exists? The Milky Way ! Our Galaxy is a collection of stars, nebulae, molecular clouds, and stellar remnants ! All bound together by gravity ! Connected by the stellar evolution cycle Determining the Shape of our Galaxy ! The number of 6400 ly stars were counted in all

directions from 1300 ly the by Sun Caroline Herschel and her brother William ! They assumed that all stars have the same brightness and that space contains no dust – these are incorrect assumptions ! They thus concluded that the Sun is at the center of the Universe - which is not true The Importance of Dust ! Dust dims and reddens starlight ! There is more dust toward the center of the Galaxy ! Consequence: We underestimate the number of stars in one direction ! We appear to be near the center, but we’re not

Us Star

Sun

Can’t see stars here (if we’re looking for blue light from them) How Do We Find the ? ! Stars aren’t all of the same brightness ! If only there was a type of star with fixed, or known luminosity ! Ideally, outside of the disk… !To see around most of the dust ! …and luminous so we can see such stars at large distances ! Old star clusters (called globular clusters...mostly red and dead stars) meet these requirements ! ~150 globular clusters are visible outside the disk Cepheid Variables ! Giant stars with masses of more

than 5 M enter periods of ⊙ variability late in their lives !Such dying stars are common in globular clusters ! CVs are unstable and pulsate ! Pulsation makes them vary in brightness ! The pulsation period is related to the star’s absolute magnitude (luminosity) ! This is an excellent way to measure distance (we can use this technique to measure distances far beyond the distances we can measure using parallax)! Finding Our Place in the Galaxy

Globular Clusters Early view: should be evenly Sun at center distributed?

Sun Globular Clusters are unevenly distributed

Harlow Shapley Finding Our Place in the Galaxy

! Harlow Shapley determined that the center of our Galaxy was in the constellation Sagittarius ! And that the center of our Galaxy was at a distance of about 12 kpc (~ 40,000 light years) ! He overestimated, because he Harlow Shapley didn’t know about interstellar dust ! Due to dust, he thought that the type of Cepheid Variable he was studying was a little fainter, and so a little farther away than is really the case

! Distance is really ~ 8.1 kpc (~ 26,500 light years) The Structure of Our Galaxy ! The disk ! All kinds of stars (30 kpc) ! Open (young) clusters ! Lots of gas and dust (8 kpc) ! The halo ! Red dwarfs and red giants ! Globular (old) clusters ! Little gas and dust Side View ! The bulge ! A mixture of the halo and the disk The Disk ! The disk of our Galaxy contains most of its visible mass ! 90% of the The disk in infrared light Galaxy’s stars ! It is where “the action” is still occurring ! Star formation, planet formation, etc.. ! It is relatively thin

! 2,000–3,000 light years thick vs. 100,000 light years in diameter Spiral Arms? ! Other disk show spiral arms ! Made of bright, blue stars and giant molecular clouds ! How do we know our Galaxy has them? ! Being inside the arms makes their spiral shape less obvious, because they’re scrunched up ! but we do observe that they are there Hints of Spiral Arms

! We plot the locations of nearby O- and B-type stars in our Galaxy ! O-type and B-type because, in other galaxies, the spiral arms consist of bright, blue stars ! We find that the stars are arranged in arms ! Our Sun is in an area between two spiral arms The Halo

! Our Galaxy’s disk is surrounded by a spherical halo of stars & globular clusters ! Red dwarfs and red giants – old stars ! Only about 2% of the number of stars in the disk ! There is some structure to the halo ! Denser towards the center ! Two sets of globular clusters ! Outer clusters – older ! Inner clusters – slightly younger (but still quite old) Our Sun orbit our Galaxy’s Center

! Could this be caused by the supermassive black hole? 2 ! Force from black hole on our Sun: F = GMBHmSUN/R

! Acceleration of Sun due to this force: F = mSUNaSUN ! 2 Thus: mSUNaSUN = GMBHmSUN/R 2 ! Therefore: aSUN = GMBH/R ! The Galactic Center is ~ 26,000 light years away (~2.5 x 1020 m) 36 ! MBH ~ 4 million M⊙ ~8 x 10 kg -11 3 -1 -2 ! G = 6.7 x 10 m kg s -15 -2 ! aSUN ~ 8 x 10 m s 2 -11 -2 ! Me on you: aYOU ~ G(85)/(9 ) ~ 7 x 10 m s Differential Rotation of the Galaxy ! The orbital periods of stars increase with distance from the center of our Galaxy ! This sounds like Kepler’s 3rd Law ! Does Kepler’s 3rd Law work? The Sun orbits at 250 km/ s or about 560,000 mph – it takes ~ 250 million years to orbit our Galaxy once! Rotation Curve Shows Hidden Mass ! Think about how we’ve used Kepler’s 3rd Law in the past to describe small masses orbiting around big masses ! Like planets around the Sun ! We always used it to determine orbits where most of the mass is on the inside of the orbit. Kepler’s 3rd law fits orbits where most of the mass is inside the orbit Rotation Curve Shows Hidden Mass ! Orbital speed actually increases slightly with distance from the Galactic center ! There must be a lot of mass in the outer parts of the Galaxy, beyond where we see most of the stars ! Only 20% of our Galaxy’s starlight is outside the Sun’s orbit

! The mass (matter ) in our Galaxy’s outskirts is dark Dark Matter

! Dark matter is of unknown origins, although several hypotheses exist ! It’s only observed by its gravitational influence on normal matter ! The dark matter in our Galaxy is in a greatly extended halo ! Up to 90% of our Galaxy’s mass is dark matter ! Our Galaxy probably contains over one- thousand-billion solar masses in total Next Time

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