Phscs 228 ch 19

LSR coordinates

(R,theta,z) cylindrical .R=distance from galactic center (0 at center) .theta=azimuthal. sun is at 0, and it increases in the direction of LSR .z=height out of the plane of the galaxy(z=0 at center)

velocity relative to the LSR

(PI, theta, z) .PI=speed in the R direction. positive is away from the galactic center .theta=positive in the direction of motion of the LSR .Z=positive toward the North Galactic Pole

Trumpler + dust

distant stars are too faint. Don't follow main sequence fitting. There must be dust between us! scattering and absorbing light to get us off

galactic latitude

b angular distance on the celestial sphere (0+-90) north or south of the galactic equator galactic equator = the great circle defined by the milky way in the sky

Herschel and Kapteyn Star Counts

basically found the same number of stars all around didn't account for dust the sun is at the center of the galaxy grindstone model

galactic coordinates

galactic latitude = b galactic longitude = l

galactic longitude

l angular distance (0-360) eastward from the direction of the galactic center in saggitarius

galaxy definition

millions to trillions of stars gas dust dark matter held together by gravity

peculiar velocity

motion of any star relative to the LSR uses (Pio, thetao, zo) where this is (0,220km/s,0)

Milky Way Bar

-barred spiral galaxy -not a spheroidal shape like initially thought -the cosmic peanut

Spiral Galaxies

-disk dominated

spiral arms

-gas and dust are concentrated along the arms -O and B stars lie directly in the arms -active star formation

galactic disk

-most luminous part of the galaxy -no sharp edges -thin disk: where active star formation is happening. Mainly pop I stars -thick disk: older stars, but more metal-rich (intermediate population)

The bulge

-pop I and II stars -no O and B stars -much older. no active star formation -no dust lanes

Local Standard of Rest

-reference frame centered on the sun, for which the average motion of stars near the sun is zero -what the sun would be doing if it were on a perfectly circular orbit -APEX=the point where we are moving relative to the LSR -ANTAPEX=the opposite point of where we are moving

Halo

-roughly spherical low density region around the galaxy -same luminosity as the bulge, but its stars are spread out over a larger region -primarily pop II stars -globular clusters and very old stars orbit at random angles to the plane of the disk

Harlow Shapely + Globular Clusters

-studied the distribution of globular clusters in the sky -they are not distributed uniformly, but are concentrated over the center of the galaxy in sagittarius -by measuring distances to the clusters, he was able to find our position in the milky way

mapping spiral arms

-use doppler shift OF HYDROGEN CLOUDS -draw concentric rings -if we are on the same ring as another hydrogen cloud, it will have the same speed as we do. Other rings will have varying degrees of blueshift

some important locations in galactic coordinates

Galactic center: b=0, l=0 galactic anticenter: l = 180, b = 0 North Galactic Pole: b = +90 South Galactic Pole: b = -90