Download Basic Observations of Stars

Stars:
Basic Observations
Distances are Hard to Measure
This took centuries of hard work following
Galileo’s first use of an astronomical
telescope, around 1600. Success came only
in 1837 (as we will learn in the next few
presentations).
But even lacking that important information,
one can learn a lot about the stars, as we will
first learn.
1. Transverse (Proper) Motions:
Changing Positions
Remember: constellations are of no real physical significance mere chance patterns. They change slowly as individual stars
move through space.
To see this, visit http://www.astronexus.com/node/28 and look
at the Animations/Travels through time under 3D Universe!
‘Proper Motions’
The apparent distribution of stars as seen on the sky can
be monitored and the ‘sideways’ motions measured as
changing directions, expressed as angles. This is called
the star’s proper motion. (To calculate the actual speeds
through space, we need to know their distances as well.)
The changes are more noticeable for nearby stars. A
nearby object can appear to ‘whiz’ across the sky even if
it is moving at modest speed. (Compare a nearby
mosquito to a high-flying jet!)
2. Stellar Colours:
Their Temperatures
From ASTR 101: the colour of a star tells us its
temperature: red = cool; yellow = middling; blue = hot.
(Objects at room temperature glow in the infrared!)
Note that the distance is irrelevant! 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!)
Cooler Stars Look Redder;
Hotter Stars Look Bluer
Human bodies, which are much cooler, glow in
the (unseen) infrared.
Hot Stars,
Cool Stars
Remember that we must consider the intrinsic light given
off by an object, not how it absorbs and reflects light
that interacts 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!
3. Stellar Spectra:
Composition, and Radial Motions
Spread the light of a star out into a spectrum.
[See ASTR 101!]
This can be done for
any sufficiently bright
star, regardless of its
distance.
The Spectrum of Vega
Note the missing colours (= “absorption lines”). Atoms
selectively absorb certain wavelengths [colours] of light,
and each atom has its own ‘fingerprint.’
Rather obviously, an absorption feature can only appear if
there are atoms of the element producing it in the outer
parts of the star. This is how we learn the composition
of the stars. (The precise details take real work!)
Radial Motions: The Doppler Shift
If the star is moving towards or away from us, the
absorption-line pattern is measureably shifted
-
toward longer wavelengths (“redshift”) if it’s moving away
toward shorter wavelengths (“blueshift”) if it’s approaching 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, since the shift is
to longer wavelengths. We still recognize the overall pattern!
Stars Move at Moderate Speeds, so
The Shifts are Very Modest
Note that a neon lamp (or equivalent) in the observatory
provides a set of ‘reference’ emission lines of known
wavelength [top]. There is no real need to directly
compare one star with another.
As Astronomers Do It
We know what gas the ‘calibration lamp’ contains, and what
wavelengths it produces. We don’t need to see the vivid colours!
Let’s Use the Doppler Shift:
Consider The Sun’s Motion
Suppose that a whole bunch of stars on one side of the sky
seem to be approaching us (at about 30 km/sec, on average),
but that the stars diametrically opposite seem to be moving
away at that same speed. What is going on?
This tells us how the sun itself is moving through the crowd of
randomly-moving stars – catching up to some in the ‘forward’
direction, leaving others behind.
Go to http://www.astronexus.com/node/28 and look at the
Orion region under Animations / Travels through time. The
sun is moving away from these stars – can you tell?
4. Not All Stellar Spectra are Alike!
Why?
A first obvious ‘guess’ might be that they differ in composition…
Not So! The Spectra Mainly Reflect
Differences in Temperature
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!
Now Consider 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 Red?
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. Rayleigh scattering is responsible
for these effects. See ASTR 101.)
Likewise the ISM
Interstellar material (ISM) can
make stars look deceptively red.
(The ISM lets most of the red light
through, but not much blue!)
The stars will 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 and nature of the ISM!
6. Yet Other Star Properties…
The study of the spectrum also reveals a
star’s
 rate
of rotation
 strength of its magnetic field
 etc
And of course we can study a star’s variability
even if we don’t know the distance.
In Summary
You can learn a great deal about the physical
properties of stars, their motions in space, and the
interstellar medium without even knowing how far
away they are!
But for real astrophysics, we need this knowledge.
How do we determine their distances? It’s not
straightforward!