Stars:
Basic Observations
Distances are Hard to Measure
This took centuries of hard work!
Success came only 1837 (as we will see)
But even lacking that important information,
one can learn a lot about the stars.
1. Transverse Motions:
Changing Positions
Remember: constellations are of no real physical
significance - mere chance patterns.
But the
Individual
Stars Move…
So: The Patterns Change –
Slowly!
We can measure the proper motions : that is,
monitor the slowly-changing positions.
Visit http://www.astronexus.com/node/28
(Look at the animations under 3D Universe!)
The changes are more noticeable for nearby stars.
2. Stellar Colours:
Their Temperatures
The colours will be unaffected by distance
provided the intervening space is clear and
transparent.
(Analogy: a red car still looks red, even when it is far down
the road!)
These colours tell us the temperatures of the stars.
Cooler Stars Look Redder;
Hotter Stars Look Bluer
An Important Distinction
We must consider the intrinsic light given off by an
object, not how it absorbs and reflects light that
can interact with the paint and pigments on its
surface.
A yellow shirt is not as hot as the surface of the
sun; your blue jeans are not as hot as the star
Rigel!
Hot Stars, Cool Stars
3. Stellar Spectra:
Composition and Radial Motions
Spread the light of a star out into a spectrum.
[ASTR 101 notes!]
This can be done for
any sufficiently bright
star, regardless of its
distance.
The Spectrum of Vega
Note the missing colours (= “absorption lines”)
What Do We Learn?
An absorption feature only appears if certain
elements are present in the outer parts of the
star.
(The atoms selectively absorb certain wavelengths
[colours] of light. Each atom has its own
‘fingerprint.’)
This is how we learn the composition of the stars.
What Else Can We Learn?
If the star is moving towards or away from us, the
absorption-line pattern is measureably shifted
-
Towards longer wavelengths (“redshift”) if it’s moving away
-
Toward shorter wavelengths (“blueshift”) if it’s approaching us.
This is the Doppler shift
The “Redshift”
The top star is at rest, so the absorption lines in its
spectrum are “where they should be.” The spectrum of
the bottom star shows that it is moving away from us.
Stars Move at Moderate Speeds, so
The Shifts are Very Modest
Astronomers Don’t Need to See
the Vivid Colours!
So the Stars Are on the Move!
What is the Sun Itself Doing?
Where are we headed through the crowd?
(Analogy: traffic on the 401, within which individual cars bob and
weave. Are we catching up, or being overtaken?)
Go to http://www.astronexus.com/node/28
and look again at the Orion region animation.
4. Not All Stellar Spectra are Alike!
Why? Do They Differ in Composition?
In Glorious Colour
Not Significantly!
These Reflect Differences in Temperature
[more on this very important point later]
5. Evidence of Interstellar
Material
Suppose you spread the light of a star out, and
see that it has an absorption-line pattern like
that of an “O” star. (See the previous panel.)
This tells you that it is a very hot star.
That’s inescapable! The spectrum doesn’t lie!
But What About the Colour?
Suppose the star delivers only a little bit of
blue light, but a lot of red light. In
other words, the star looks red.
Doesn’t this tell you that the star is cool?
Not Necessarily!
[The Sun Looks Red Every Evening!]
Why So?
The colour of the sun can be affected by
intervening material. (We see it low in the
sky , through lots of the Earth’s atmosphere.)
Likewise:
Interstellar material
can make the stars
look deceptively red.
The Interstellar Medium (ISM)
As noted, stars look redder than they really are.
But they also look fainter than they should.
This gives the impression that they are farther
away than we might think.
That’s a problem, if we are trying to ‘map out the
galaxy!’ We have to understand the distribution,
amount and nature of the ISM!
6. Yet Other Star Properties…
The study of the spectrum also reveals its
 rate
of rotation
 strength of magnetic field
 etc
And of course we can study a star’s
variability even if we don’t know the
distance.