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 (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 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. 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 and look again at the Orion region animation. 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! 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.