Download Galaxies

Galaxies
Star Motion
• Stellar motion is measured with
respect to the Sun.
v 2  vr  vt
• The radial speed of a star is
determined through the Doppler
shift of the spectrum.
  0
vr 
c
0
• Proper motion m measures the
speed perpendicular to the
radius.
– Units are arc sec/year
– Use parallax p
2
2
tan   vt / vr
vt  d sin m  md
vt  4.74md  4.74(m / p) km/s
Local Standard of Rest
• The Sun orbits the galaxy.
m
m
vr
vr
• This can be corrected by
assuming a circular orbit as a
reference frame.
– Dynamical standard of rest
• Deviations in apparent motions
of stars reflect solar motion.
Galactic Motion
• Motion in the galaxy comes
from the motion of stars and
clusters.
– The sun has a period of 230
million years.
• Atoms of hydrogen absorb and
emit radio waves with a
wavelength of 21 cm.
– Measure velocity of gas
clouds by the doppler shift
Oort’s Model
•
•
•
Calculating galactic motion is
difficult from the Sun.
Assume circular orbits.
– Orbital speed Q, w
– LSR speed Q0, w0
– Radial distances R, R0
– Distance d
– Longitude l
– Angle of velocity a
Can be used to distinguish
rotational patterns.
vr  Q cos a  Q0 sin l
sin l / R  cos a / R0
w  Q/ R
vr  R0 (w  w0 ) sin l
vt  Q sin a  Q0 cos l
sin l / R  cos(l  a ) / d
vt  R0 (w  w0 ) cos l  dw
Differential Velocity
4 2 3
T 
R
GM
2
4 2 2 4 2 3
R 
R
2
v
GM
T
2
R
v
v
1
R
• If most of the galactic mass
were at the center then there
would be Keplerian orbits for
the stars.
– Inconsistent with
observation
Galactic Nucleus
• The nucleus of the Milky Way
is known to be in the direction
of Sagitarrius.
Mass of the Galaxy
• The orbit of clusters can be used to
estimate the mass of the galaxy.
• This is the same method used for
planets and binary stars.
• The Milky Way galaxy has a mass of
1 trillion (1012) M.
• Most of the mass is not at the center.
infrared image
Radiating Gas
• Dust obscures visible light from
the center of the galaxy.
• Radio waves are good for
viewing the center.
Mass at the Center
• The motion of stars near the galactic center gives the mass
of the galactic nucleus.
• Infrared images of the stars show the motion.
• The mass is 2.6 million (106) M.
Problem with Orbits
• There are 200 billion stars in the Milky Way.
• Stars, gas, and dust are considered visible matter.
• We can also estimate the mass in black holes.
• The visible mass does not explain the orbits of stars in the
galaxy.
• The apparent mass is much larger (10 times).
Many Galaxies
• Any large group of stars
separated by 100’s of thousands
of light years from other groups
is called a galaxy.
• There are over 200 billion
observable galaxies.
• There are a wide variety of
sizes and shapes.
• Galaxies are classified by their
shape.
Spirals
• Spiral galaxies have a dense nucleus and multiple arms.
• The arms can be tight or wide apart.
• Spirals are usually large and have active star formation.
Barred Spirals
• Some spiral galaxies show a distinct bar-shaped nucleus.
• The central bar is anchored to two large arms.
Elliptical Galaxies
• Elliptical galaxies are round or ovoid.
• They are often small compared to spiral galaxies.
• Ellipticals have little dust and new star formation is rare.
Irregular Galaxies
• Irregular galaxies have illdefined shapes.
• Like ellipticals they are usually
small.
• Unlike ellipticals they have
active star formation.
• Irregular galaxies are usually
seen near larger galaxies.
Hubble’s System
• Edwin Hubble used the shapes
to label galaxy types.
• E: elliptical galaxy;
the number indicates how
stretched out the shape is.
• S (or SA): regular spiral galaxy;
SB: barred spiral galaxy;
the small letter indicates how
spread out the arms are.
Galaxy Formation
• Like stars, galaxies start with gas.
• The first formation was about 13 billion years ago.
• Gravity pulled small gas clouds together—with enough
density, stars form.
• These first galaxies were elliptical shapes.
Close Encounters
• Galaxies are attracted to each other by gravity.
• If two ellipticals pass each other gas and stars are pulled.
• The galaxy is distorted as it rotates.
Star Ages
• Ellipticals generally have old
stars (type II).
• The center of spiral galaxies
also have type II stars.
• This suggests that they were
formed first.
• The disk of a spiral galaxy has
type I stars that are young.
New Star Regions
• As an elliptical passes through, the stars do not collide.
• The gas and dust in the two galaxies collide.
• The increased density and gravity creates new stars.
Ring Galaxies
• One galaxy may pass through
another.
• The collision forms a ring of
stars and gas.
• Gravity can pull the stars in,
forming spiral arms.
Forming Spirals
• Computer simulations are used
to see the effect of collisions.
• A near miss forms a spiral
galaxy (top sequence).
• A direct hit forms a ring galaxy
(bottom sequence).
History in Shapes
• Spiral galaxies are created by
collisions of galaxies.
• There are many spiral galaxies,
so collisions are common.
• Irregular galaxies occur during
collisions.
• This lasts about 100 million
years.
• The spiral will last for billions
of years.