University of Wisconsin – Eau Claire

Continuing Education

Dr. Nathan MillerDepartment of Physics & Astronomy

To the Stars and Beyond

WELCOME BACK!

Appearance and motions of night sky

objects Visit to the planetarium to see sky motions

in 3D (we will walk over together) Telescopes: design and basic use The Lives of the Stars The Universe and the Big Bang Life in the universe and planets where it

may be found

Main topics of Course

The Stars

How bright?How big?

How massive?How hot?How old?

What are they made of?What causes them to shine?

How far away?

First Question: How Bright?

• Hipparchus – 2nd cent. BC. Put many stars in 6 brightness categories

• 1st magnitude = brightest• 6th magnitude = dimmest seen

• Magnitude 5 star is 100 times dimmer than Magnitude 1 star

• Sun = Mag -26• Brightest star = Mag -1• Dimmest star you can see = Mag 6• Amateur Telescope = Mag 12• Hubble Space Telescope = Mag 25

But raw brightness doesn’t tell you much about stars themselves.

i.e. A 100-watt bulb held next to your eye appears much brighter than a street light. But which is the more powerful bulb?

You need the distance

To find Distance, use Parallax

Parallaxes are small.

• A star with a parallax of 1 arcsecond would be at a distance of 1 parsec (=“parallax second”)

• No stars are this close

Absolute magnitude:How bright would the star be if it

were at 10 parsecs?

A star with a brighter absolute magnitude is really putting out more

light than a star with a dimmer absolute magnitude.

• Apparent Brightness• Absolute Brightness (“luminosity”,”Absolute magnitude”)• Distance

• Give me any two and I will tell you the third

To study color better, use a prisim to spread out starlight into colors

Star’s colors are caused by “blackbody radiation”

• http://phet.colorado.edu/en/simulation/blackbody-spectrum

The Hertsprung-Russell Diagram

- The Rosetta Stone for StellarAstrophysics

What Russell needed to know (1913):

Spectral types of the nearest stars (Spectra)

Distance of nearest stars (Parallax)

Brightness of nearest stars (photography)

Use Distance and Brightness to get

Intrinsic luminosity

The basic Hertsprung Russel Diagram:

Plotted on the graph, most stars are on the Main Sequence

Every square meter of a hot thing emits much more light that a square meter of a cold thing

So the main sequence stars are all roughly the same size.

All the nearest stars plotted:

Some stars do not fall on the Main Sequence: Giants and White

Dwarfs

• If something is hot but dim, it must not have many square meters small

• If something is cool but bright, it must have many square meters huge

So we can find the sizes of stars:

Draw lines of equal radius on the HR diagram:

Which of the directions in the following HR diagram correspond to an object which is

contracting?

• A. A.• B. B.• C. C.• D. D.• E. More than one of the above

Star Clusters• 2 kinds –

• Open Clusters – young, in galactic plane

• Globular Clusters – old, swarm around galaxy

Pleiades Open Cluster

Open Cluster Near Galaxy Center

Open Cluster

M38

Globular Cluster M2

Globular Cluster M15

Clusters and Stellar Evolution

In each cluster:• Stars all made at nearly same time• Stars all the same distance from Earth • Stars in cluster that look brighter really are

brighter

Zero-Age Main Sequence (ZAMS) –

Position on HR diagram where stars begin H fusion in core

Core slowly depletes H fuel core shrinks

core heats up higher fusion rate

star gets slightly brighter

Cluster Main Seq.Turnoff• Bright, high mass stars evolve first

• In older clusters, these stars have started to “turn off” the main sequence

Which is Older?

A. M41

B. NGC 752

Evolution of Individual Stars

Brown Dwarfs

Not enough mass to start fusion, so never really a true star

Still glow through gravitational contraction.

Very Low Mass Stars

• Universe not old enough for them to have evolved much are still on Main Sequence

• When they do evolve, they will move left on the HR diagram to be White Dwarfs

Sun-like Stars

• Eventually, they run out of H fuel in their cores

• Core shrinks until it is supported only by “degeneracy pressure”

• H burning continues in shell around core

Sun will become huge gravity less strong on outer

layers

Outer layers drift off to become “Planetary Nebula”

Core left behind is “White Dwarf”

As they cool, white dwarfs get:• A. Quite a bit bigger• B. Quite a bit smaller• C. They remain about the same size

Evolution of Sun (click on image)

TheRing

Nebula

THE EVENTUAL FATES OF HIGH-MASS STARS

2 types of Supernovae

Will concentrate on Type II – Explosion of a massive Star

Type Ia – involves a white dwarf in a binary system

Star burns H, then He, then heavier and heavier elements up to Iron

After core is fused to iron, star can get no further energy from fusion

No fusion in core core collapses material “bounces” outward

Tycho’s SN -- 1572

Kepler’s SN-- 1604

The last Supernovae Observed in our Own Galaxy

Supernova 1987A

Nearest supernova observed in modern times

Not in the Milky Way, but right next to us in the Large Magellanic Cloud

Cas A

S147

Veil Nebula

The Crab Nebula

Close – 2 Kpc away Supernova observed by Chinese

astronomers in 1054 AD (we know from expansion velocities)

In void in ISM – did not sweep up material (that’s why the edge is not well defined)

Crab in X-rays

chandramovie_sm.mov

Neutron Stars:

Core of massive star after supernova

Protons and electrons squeezed together, only neutrons remain

Radius of about 12 kilometers

Pulsars

Point like objects that “pulsed” quickly and rapidly in radio light

Discovered by Joycelyn Bell and Antony Hewish in late 60’s

The Crab Pulsar

We observe a pulsar from earth. We contact an alien civilization outside the solar system.

Will they see the object as a pulsar?

A. Yes B. No C. Maybe – it depends

Black Holes:

The most massive remnants cannot support themselves even after crushing all their material into neutrons

They collapse into black holes

Black holes are black because their escape velocity is greater than the speed of light (300,000 km/s) – i.e. not even light can escape.

(For comparison the Earth’s escape velocity is 11 km/s)

Schwarzschild Radius how “big” a black hole is

Rs = 3M (Rs in km, M in Solar Masses)

Schwarzschild radius locates “event horizon”

Anything crossing the event horizon will never be seen from again.

How do you detect black holes if you cannot see them?

Matter or gas rotating fast around a small point indicates mass must be extremely concentrated

Where does the material come from? Often a binary companion.

Which star initially had more mass?A. Black holeB. Companion StarC. It could be either – no way to tell

If the Sun were instantaneously replaced by a 1 solar mass black hole, what would

happen to the Earth? A. It would rapidly spiral into the black

hole B. It would continue merrily along its

orbit