Transcript Galactic Structure

Galactic Structure
Motion of the Stars
• The gravity of stars in the galaxy causes stars to move.
• This motion can be separated into two parts:
Radial motion is motion towards or away from the Earth.
Transverse motion moves across the path from the Earth.
Sun & Earth
observed star
radial motion
transverse
motion
actual motion
Proper Motion
• Proper motion measures a
star’s position after
adjusting for the Earth’s
motion around the Sun.
– Combine with distance
– Used to find transverse
motion
• Doppler shift is used to
find the radial motion.
Barnard’s Star is close to Earth
Runaway Stars
• Some stars appear to move
away from a common
point.
AE Aurigae
53 Arietis
Mu Columbae
– Past explosion
• 53 Arietis, AE Aurigae,
and Mu Columbae would
all be in the area of the
Orion Nebula about 3
million years ago.
Populations
• Stars are segregated by color in other galaxies.
– disk stars more blue – population I
– halo stars more red – population II
• Similar segregation is present in clusters in the Milky Way.
sun
side view
Cluster Types
Type I Clusters
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Open clusters
Hot young stars
Lots of gas and dust
Abundant in heavy elements
 Active star formation
Type II Clusters
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Globular clusters
Old red stars
No gas and dust
No heavy elements, just
hydrogen and helium
 No star formation, just old
stars
Galactic Motion
• Gravity applies to stars in
the galaxy.
• The motion of the globular
clusters indicates the center.
• The stars in the disk also
orbit the center.
– Sun orbits in 230 million
years
Fountains
• Inside the Milky Way
there are areas moving
into and out of the disk.
– Clusters pass through the
disk
– Stars and black holes
Galaxy Formation
• Like stars, galaxies start with gas.
– First formation about 13 billion years ago
• Gravity pulled small gas clouds together—with enough
density, stars form.
– Galaxies merged to form the Milky Way