Stars, Galaxies, and Cosmology

Extra-Solar Planets• Meaning planets around other stars.• So far over 400 planets around over 300 stars have

been detected by Doppler shifts in starlight.• As a planet orbits its sun, it tugs one way then the

other.• The subsequent movement of the star causes the

Doppler shift in its light.• So far, only large planets can be detected.• But one has recently been spectrographed!• When we find O2, BINGO!

A Lot of Nothing

• After the Sun, the nearest star is 4.3 light years away.

• A light year (LY) is about 5.87 trillion miles, or the distance light travels in a year

• Put another way, @ 65 mph, it would take about 10.3 million years driving non-stop to get there (and at these gas prices…)

Not Exactly Empty

• Giant Molecular Clouds exist between the stars.

• Very thin dust, more like cigarette smoke, and lots of hydrogen.

• Our cloud is called The Local Bubble.• These clouds tend to redden the light from

stars, much like our air pollution reddens the sunset.

Nursery• These clouds are “thicker” in

certain parts of the galaxy, providing a nursery for new stars.

• When a pressure wave travels through these clouds, the gas (H) and dust compacts enough so that gravity can slowly compress the “clumps” into protostars that eventually become solar nebula that then become_____________.

Bok Globule

One Quality to Rule Them All

• The single quality of a star that most determines its existence and its fate is its mass, both initially and at the end of its life cycle– A reminder: when we talk about the life and

death of a star, we are not talking Hollywood; our stars have more character but are not alive

Mass/Lifetime Line

BrownDwarf

RedDwarf

YellowDwarf

Giants

0.085Msol 0.80Msol 1.4Msol

Chandrasekhar Limit

~3Msol

12 Tyr 20Gyr 5Gyr 10Myr

Stellar Types (simplified view)

• Stars < 8.5% the Sun’s mass (Ms) fail to start the fusion process—called Brown dwarfs.

• Stars > 10% Ms but < 80% Ms are called Red Dwarfs and exist for up to 200 Billion years.

• Stars between 80% Ms and 140% Ms are like our sun, and live about 10 billion years.

• Stars* between 140% Ms and 300% Ms live only about 10 million years and explode when they die.

• Stars* >300% Ms collapse completely out of the Universe and become Black Holes.*at the end of their fusion epoch

So….

• What conclusion can you draw about the mass of a star predicting its lifespan?

• The more massive a star, the shorter it “lives”.• This is because larger, more massive stars are

much hotter and burn (fuse) their nuclear fuel much more quickly.

• Like a Corvette burning gas more quickly than a Prius.

The Death of Sun-like Stars

• After about 5 billion more years, the H in the core will run out.

• The core will collapse, and the outer layers will be blown outward, engulfing the inner planets.

• Eventually the core will collapse so much that the temperature will rise to 100 million K; then Helium will fuse into Carbon.

“Shine On You Crazy…”

• Eventually though, the helium will run out, and the star, now only the size of a large rocky planet, will be made of hot, highly compressed carbon, cooling off for billions of years as a White Dwarf, shining by its heat alone.

• What do you get when you compress hot carbon for a long time?

KaBoom Stars• The more massive stars that

explode fuse elements all the way down to iron, and in their explosion fuse elements even heavier.

• In fact, all elements, in your body, your car, your pet cat Fluffy, except for hydrogen and some helium, were generated in this stellar explosion, called a Super Nova.

• The remnants of these explosions is a tiny dense object known as a Neutron Star.

Supernova 1987A

• Exploded in the Large Magellanic Cloud 167,000 years ago– Light finally

arrived in 1987• As much light

billions of 1013 suns!

You Can Check In…• But you can never check out of a

Black Hole.• When a supermassive star (>10

Msol) runs out of fuel and tries to explode, gravity thwarts the kaboom and squeezes the star into zero volume.

• This has the effect of bending the space around it, like taking the floor and wrapping it up in a ball around you.

• Called Black because no light escapes, and Hole because it is made out of nothing, like any good hole.

An Extended Family of Stars

• The HR diagram• Stellar output on

the vertical axis• Temperature on

the horizontal• O, B, A, F, G, K,

M are the Spectral Classes

Clusters

• When an interstellar cloud is impacted by a shock wave, many stars ultimately form, depending on the size of the cloud.

• These masses of stars are known as Clusters.

Types• Globular Clusters can contain as many as a million,

older stars (red dwarfs), and most are found in the halo outside the plane of the galaxy.

• Open clusters contain about 1-10 thousand young, hot stars, and are mostly in the spiral arms of the galaxy. The Pleides (7 sisters) is one such example.

• Associations are very poor in stars, about 100 or so young, hot stars, located in the plane of the galaxy.– Actually disassociating

Globular and Open

Altogether

• The stars and their planets, the groups of stars we call clusters, the interstellar dust and gas clouds, all make up a structure known as a galaxy.

• Our galaxy is known as the Milky Way.

Vitals• Our galaxy has a spiral shape with two bright and one

faint arms.• The core or nucleus of the Milky Way contains a

supermassive Black Hole.• The galaxy is about 100,000LY across, 15,000LY at

its thickest, and contains about 100-500 billion stars.• Our solar system is about 30,000LY from the core.• We orbit the core about once every 240 million years.

Other Kinds of Galaxies

• The most common is the elliptical galaxy, shaped sort of like a big egg.

• There are spiral and barred spiral galaxies,• Irregularly shaped galaxies also exist, though fewer

in number.• A galaxy’s shape is determined by its rate of spin,

and if it has been subject to any collisions or mergers.

• These all contain 100 billion stars or more, and there are 100 billion galaxies out there!

For Classification Purposes Only; no evolution is implied

Grouping

• Galaxies also form groups based on gravitational attraction.

• The nearest galaxy to us is Andromeda, about 1.5 million LY away.

• This and about 30 other galaxies form the Local Group, about 5-10 million LY across.

Bigger..

• In turn, our Local Group is part of a larger conglomeration known as a galactic cluster.

• Our local cluster is known as the, well, the Local Cluster.

And Bigger…• Galactic clusters are arranged in huge filaments called

superclusters that stretch across vast regions of space, containing millions of galaxies.

the Virgo Supercluster

(part of) Large Scale Structure

The Big Bang

• Neither big nor a bang.• Based on the movement of galaxies observed

today, astrophysicists can “run the clock backwards” and estimate that the Universe began about 13.7 billion years ago.

• The event is now called the Big Bang; however, it took place in an infinitesimally tiny space and there was no sound.

And it continues..

How do we know this?

• Edwin Hubble ~1929 observed all distant galaxies moving away from us, indicating expansion

• Einstein’s General Theory of Relativity mathematically predicts the curvature of space, like in a Black Hole.

• This curvature is observed and measured regularly because it changes as time passes and the Universe “flattens”.

• Therefore, mathematics can predict when the curvature was infinitely wrapped up.

Evidence?

• If the Universe was infinitely wrapped up, then due to thermodynamics it would have been infinitely hot.

• As it unwraps and flattens, it should cool to a specific temperature, based on known cooling processes.

• Also, high temperatures would have emitted radiation, which would have changed frequency as the Universe cooled.

Found in 1963

• Two scientists in NJ accidentally found this cosmic microwave background radiation (CMB) while experimenting with satellite communication.

• Its frequency and the associated temperature matched the predictions.

• Certain details need to be refined, but the idea of a miniscule, impossibly hot start to our Universe is widely accepted by Astrophysicists.

WMAP: Map of CMB

Where We Are Going: Possible Fates of the Universe

1 2 3 4

Critical Mass

• Einstein’s theories say that mass curves space.

• If enough mass exists, the Universe will eventually curve back in on itself, causing a “Big Crunch”. (1 on previous page)

• If too little mass exists, things will flatten and expand forever, resulting in the “Heat Death of the Universe”. (2 & 3)

Current Prediction

• Insufficient mass has been detected to ‘close’ the Universe

• In addition, observations show the Universe is not only expanding but accelerating (4)

• Current predictions are for the Universe to expand for about 10,000 million, million, million, million, million, million, million, million, million, million, million, million years, at which time it will be an empty, cold place.

Plenty of Time to Study for the Final Exam!