The Stellar Population

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The most important property of a star is its mass More massive stars have stronger gravity, which

increases pressure and temperature to the point that fusion can start

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A higher mass and temperature leads to a higher luminosity A higher mass also needs more support against

gravity This support comes from increased energy

generation from fusion This is another way to see why massive stars are

more luminous

424 TRL

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More luminous stars use up more energy in shorter period of time Although they have more fuel, they use it so

quickly that massive stars don’t live very long

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Magnitudes Old habits die hard Use logarithms

This is called apparent magnitude

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F

Fm

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Apparent magnitudes depend on our observation, but don’t tell us about the true nature of the star

For this we use absolute magnitude (M)

5)log(5 dMm

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Stars all have a very similar composition 70% Hydrogen 28% Helium 2% Everything else

How do we know all this? Composition from line spectra

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We looked at other possible sources of energy besides fusion Chemical burning and gravitational collapse

cannot produce enough energy We don’t have the right materials for fission and

antimatter power

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There was a problem with fusion Not detecting enough neutrinos After a lot of work, the problem was explained

Did not understand neutrinos Great example of how science works

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Began to talk about stellar populations The Sun is not typical or common, but it also

doesn’t really stand out Most stars are low mass, cool, and long-lived Massive, hot stars are very rare

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Spectral ClassesO B A F G K M

Oh Be A Fine Girl/Guy Kiss Me

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Demo Day! Temperature and pressure relationship Lenses and mirrors Photo-electric effect Polarization

After this, we will talk more about stellar populations

Pressure Temperature Relationship

The Energy of The Sun

The Energy of the Sun

Photoelectric Effect

Polarization

Back To Stellar Populations

Let’s take a quick look at some spectra

Comparing Spectra

Stellar Classification

Now that we have our groups, we want to try to understand why they are the way they are

What do you think is responsible for the different spectra?

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1. Different composition

2. Different environments

3. Different temperature

Temperature

The main difference between different classes is the temperature of the star It all has to do with the blackbody curve and

energy Our old applet friend

Temperature

Each type of absorption line that we saw has a certain energy at which it is most efficiently produced Some ions and molecules are destroyed in the

intense radiation of the hottest stars

Other Observables

Now astronomers had a way of measuring temperature, though it was not well calibrated

They could also measure apparent brightness But this is not intrinsic How can we measure intrinsic brightness

Luminosity

Two possibilities Measure the distance to the stars (hard) Measure apparent brightness for stars at a fixed

distance (not as hard)

Star Clusters

Astronomers knew of objects whose stars were all at a fixed distance

The Hertzprung-Russell Diagram The next thing to do is to combine this

information Luminosity Temperature

What do you think we will see?

The Hertzprung-Russell Diagram

The Hertzprung-Russell Diagram A high luminosity and low temp indicate a

very big star The main line of stars are called the main

sequence Temperature and luminosity increase steadily (as

does radius) A high temp and low luminosity indicate a

very small star

The H-R Diagram

The H-R Diagram

But is this a special circumstance, or is it true of all populations of stars

Need to measure the distance to stars

Parallax

Hold your thumb, and switch between one eye being opened and the other closed What happens? Why?

Parallax

The same thing happens for stars, but the are so far away that the change is small Hard to detect

Parallax

Measuring Parallax

To measure parallax, we need to know positions very accurately This was a big project here at UVa

This is also where we get the unit of parsecs from At a distance of 1 parsec, a star has a parallax of

one arcsecond

Back To The H-R Diagram

The same trend is seen This is a real phenomenon Stars come in a wide range of temperatures,

sizes, and masses As a dramatic example of this, here is a short

video

Consider It…

With nothing but spectral lines and luminosity, we have discovered something amazing There is a rich variety of stars in our galaxy The next question is, why?

Alien Observer

If an alien came to Earth, it would see: Young, tiny humans that constantly poop Larger, rambunctious creatures that fear cooties Larger still beings that are obsessed with bad

music Lots of large beings that work all day Smaller, frailer beings that complain about all the

other ones

Alien Observer

This alien could conclude two things Humans are created in an amazing variety of

sizes and they all are annoying Humans grow and go through a lifecycle, and are

annoying at each stage Either way, the end result is the same…

Which is it for stars?

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1. They are born with this variety

2. They go through a life-cycle

The Lives of Stars

The Lives of Stars

It is best to begin at the beginning, with the birth of a new star As we will see, stars spend most of their lives

doing relatively nothing But when things do get interesting, they get really

interesting

Star Formation

These images are beautiful examples of stellar nurseries Let’s step through what is actually happening

Star Formation

Stars start off as huge collections of gas and dust These clouds are very cold, and are called giant

molecular clouds As the name suggests, they contain molecules

Giant Molecular Clouds

Note that the temperature and fact that they contain molecules are related Molecules cannot exist in very hot environments

GMCs can contain up to millions of solar masses

Giant Molecular Clouds

Although they are nothing more than giant balls of dust, they are very important In our galaxy, and galaxies like it, GMCs are the

only site of significant star formation

Giant Molecular Clouds

Think about it… They are very cold (10s of degrees Kelvin) Roughly what should the peak wavelength be? Are they black bodies? Why do they glow?

Giant Molecular Clouds

GMCs glow because there are already some stars present These stars heat the gas The gas gives off emission lines, just like our

vacuum tubes

Back To Star Formation

How do stars actually form? Need a concentration of mass Some regions of the cloud are more dense than

others These areas have a stronger gravitational field They attract more mass to themselves

Star Formation

This is a runaway process As the pocket gains mass, it increases its gravity This attracts even more mass

Think about it…what will happen when the pocket gets massive enough

Star Formation

1. As the star gets more massive, the gravity and pressure increase

2. This increases the temperature

3. Eventually, densities and temperatures are high enough to begin fusion

Movie time!

Proto-stars

Once fusion begins, we officially have a new star Can we see the stars before this happens? Yes…remember gravitational collapse

A Brand New Star

What do you think the conditions around these new stars will be like?

Disks

There is lots of left over material around the star Essentially, the star lives in a dirty neighborhood

This material forms a swirling ring of gas around the star This can make them hard to see

IF this material is pristine, that is it has never been used by a star before, do you think Earth-like planets can form from it?

50%50%1. Yes

2. No

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Stellar Disks

As we will see, the materials need to make Earth-like planets can only be made by stars

If this material were pristine, there would be nothing to make Earth-like planets out of

So to get to planets, we need to keep following our the life of our young star Next time…