Astronomy 162: Introduction to Stars, Galaxies, & the Universe Prof. Richard Pogge, MTWThF 9:30

Lecture 6: The Motions of the Stars

Key Ideas

The stars are in constant motion.

Observed Motions:

Proper Motions (across the sky)

Radial Velocity (towards/away from us)

True Space Motion

Combination of radial velocity, proper motion, & distance.

The "Fixed Stars"

In reality, the stars are in constant motion.

The great distances to the stars means that their apparent motions across the sky are very small during a human lifetime.

Proper Motions

• Typical proper motion is ~0.1 arcsec/year.
• Largest: 10.25 arcsec/yr (Barnard's Star).

This is the projection onto the sky of the star's true motions through space relative to the Sun.

Proper motions are Cumulative.

The longer you wait, the greater the apparent angular motion is

Modern measurement of proper motions:

• Compare images of the sky taken 20 to 50 years apart.
• Measure how much the stars have moved relative to distant background objects (usually galaxies or quasars).

Example:

Consider a star with a proper motion of 0.1 arcsec/year:

• After 1 year: star has moved 0.1 arcsec
• After 10 years: star has moved 0.1x10 = 1 arcsec
• After 100 years: star has moved 0.1x100 = 10 arcsec

Since the smallest angle the eye can discern with great care is a few arcminutes (1 arcmin = 60 arcsec), it can take many millennia for the constellations to noticeably change shape.

Discovery

Proper motions were first noted by Edmund Halley in 1718 for three bright stars: Sirius, Aldebaran, and Arcturus, by comparing his measurements of their positions to those of Hipparchus of Rhodes (300BC). In all, it took 2000 years for the motions to build up to the point that they became apparent to naked eye observers.

Case Study: Proper motions in the Big Dipper

Here is a movie showing 200,000 years of proper motion in the Big Dipper, including faint stars. Notice how they all tend to move in different directions, but some (like 5 in the Dipper) have common motions...

Proper Motion depends on the Distance

• More distant stars tend to have smaller Proper Motions
• Can usually only measure proper motions for stars within about 1000 parsecs of the sun.

But...

Distance is only part of the effect!

A small proper motion does not always mean a large distance!

Example:

Stars moving exactly towards or away from us will show no proper motions!

Radial Velocity

Radial velocities are measured using the Doppler Shift of the star's spectrum:

• Star moving towards Earth: Blueshift
• Star moving away from Earth: Redshift
• Star moving across our line of sight: No Shift

In all cases, the Radial Velocity is Independent of Distance.

True Space Motions

To find the true space velocity of a star, we need to break its motions into two velocity components:

Radial Velocity (v_r)

Measure this using the Doppler Shift of its spectrum.

Tangential Velocity (v_t)

Measure this from its Proper Motion and Distance:

where:

m = Proper Motion in arcsec/yr

d = Distance in parsecs

The formula above gives v_t in km/sec.

Each of these velocities forms the legs of a right triangle with the true space velocity (v) as the hypotenuse.

We can then use the Pythagorean Theorem to derive the True Space Velocity (v):

To estimate the true space velocity, you need to measure three observable quantities:

• The Radial Velocity
• The Proper Motion
• The Distance

The last is often the most difficult to measure (as always).

Why measure the space motions?

Use statistics of the motions to find:

• Motion of the Sun through nearby space (towards the constellation of Hercules)
• Local rotation of the Galactic Plane
• Identify odd-ball stars that move "peculiarly" relative to otherwise similar stars.

Stellar motions are an important tool for studying the structure of our home galaxy, the Milky Way.