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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 3: Our Neighborhood: Nearby Stars, Stellar Populations, and the Galaxy


Key Ideas

Space is very empty
There are only a handful of stars within a few parsecs
--Closest is the triple system Alpha Centauri A,B, and Proxima Centauri
There are ~0.1 stars per cubic parsec near the Sun
Most stars are low-mass M dwarfs
The Sun lives about 8,000 parsecs from the center of the Galaxy
The center of the Galaxy is called the Galactic Bulge
--The bulge is denser and older, but contains a lot of stars
There are 200-400 billion stars in the Milky Way.
There are ~100 billion Galaxies in the observable Universe.
There are a lot of stars in the Universe!


Where are the stars?

Stars near the Sun:
--How far away are they?
--Which kinds of stars are the most common?

Stars in the Galaxy
--How are the stars distributed in the Galaxy>
--How many stars are there in the Galaxy?

Stars in the Universe?
--How many Galaxies? How many stars?


Won't You Be My Neighbor?

There are no stars within one parsec!

Closest Star

Closest star to the Sun is Proxima Centauri
--Constellation Centaurus
--Too faint to be seen with the naked eye.
--100 times fainter than the eye can see!
M5.5 dwarf star
--Mass=0.12 Solar Masses
--Radius=0.15 Solar Radii
--Luminosity ~0.01 of the Luminosity of the SUn

How Far Away Is It?

Distances of 1.30 pc (4.22 ly)
270,000 times further away than the Sun
If the Sun were 3.5 cm in size, Proxima Centauri would be ~1000 km away!

Getting There

If we traveled at 25,000 mph, it would take over 100,000 years to get there

Space is very empty, and stars are very far away and very far apart.

Alpha Centauri A & B

Distance = 1.34 pc (4.37 ly)
Binary star system (triple system with Proxima)

Alpha Cen A:
--G2 V (like the Sun)
--Mass = 1.1 Solar Masses
--Radius =1.2 Solar Radii

Alpha Cen B:
--K1 V
--Mass = 0.9 Solar Masses
--Radius = 0.87 Solar Radii

Alpha Centauri A & B and Proxima Centauri are potentially habitable


The Local Solar Neighborhood

Local Stellar Populations

About 70 stars within 5 parsecs.

About 0.1 stars per cubic parsec

Main Sequence Stars

Fraction of main-sequence stars:
O Stars -- 0.001%
B Stars -- 0.1%
A Stars -- 1%
F Stars -- 2%
G Stars -- 7%
K Stars -- 15%
M Stars -- 75%

The majority of stars are M dwarfs.

Thin disk

Above about 100 pc, number of stars begins to decline in certain directions


The Milky Way

Diffuse band of light crossing the night sky
Our names are derived from Greek and Latin:
--Greek: Galaxias kuklos = "Milky Band"
--Latin: Via Lactea = "Road of Milk"

Properties of Our Milky Way

A flattened disk of stars with a central bulge
Sun is ~8 kpc from the center in Sagittarius
~30 kpc in diameter and ~1 kpc thick
Galactic Center and much of the disk is obscured by dust in the plane of the Galaxy

The Galactic Bulge

Roughly spherical population of stars
Most stars are older than those near the sun
--~10 Gyr old (almost as old as the Universe!)
Much denser
--~100 stars per cubic parsec
Lots of stars
--Tens of billion stars!

There are over 200-400 billion stars in the Milky Way.


Nearby Galaxies

Closest Galaxies are the large and small Magellanic clouds (LMC and SMC)

-- Large Magellanic cloud only has ~one tenth of the stars in the Milky Way

Andromeda Galaxy

Closest massive Galaxy like the Milky Way
Slightly bigger than the Milky Way

Local Group of Galaxies

Dominated by the Milky Way and Andromeda
Dozens of additional small Galaxies like the LMC and SMC


It's A Big Universe

How Many Galaxies?
The Hubble Ultra Deep Field (HUDF) was only one tenth the diameter of the Moon.
Would take ~80 HUDFs to cover the Moon.
Would take ~10 million HUDFs to cover the sky!
HUDF has 10,000 Galaxies
The total number of Galaxies is 10,000 times 10 million ~ 100 billion.

How Many Stars in the Universe?
~100 billion Galaxies
~100 billion stars per Galaxy
Total number of stars in the Universe is: 100 billion times 100 billion ~ 10 billion trillion!


See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

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