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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 2: Stars: Habitable Zones, Lifetimes, and Other Considerations

Key Ideas

The Main Sequence is a Mass Sequence
--Higher mass stars are hotter, bluer, and more luminous
Habitable Zones
--Higher mass stars have broader habitable zones further from the star
--Higher mass stars have shorter lifetimes
--Habitable zone moves outward more quickly for higher mass stars
Other Considerations
--Tidal Locking, Flares, UV radiation

Main Sequence stars are the best places to look for life.

Which are the best places to look for life?

Conditions conducive to life:
--Stable, long-lived energy source
--Elements of life (water, carbon, etc)
--Benign environmental conditions
--A location for life to arise


Stars which stably generate energy by fusion of H into He

Which are the best places to look for life?

Main-sequence stars with rocky planets in their habitable zones.

Which are the best stars to look for life?

Factors we want to consider:
--Habitable Zones
--Tidal Locking
--Other peculiarities

Main Sequence Stars

The Main Sequence is a Mass Sequence

The location of a star along the M-S is determined by its Mass.
Low-Mass Stars: Cool & Low Luminosity
High-Mass Stars: Hot & High Luminosity
Result of the Mass-Luminosity Relation:

O Stars

O5 Main Sequence Star
Mass = 58 Solar Masses
Temperature = 46,000 K
Radii = 14 Solar Radii
Luminosity = 800,000 times Solar Luminosities

A Stars

A0 Main Sequence Star
Mass = 2.6 Solar Masses
Temperature = 9,600 K
Radii = 2.3 Solar Radii
Luminosity = 63 Solar Luminosities

G Stars

G2 Main Sequence Star
Mass = 1 Solar Masses
Temperature = 5.700 K
Radii = 1 Solar Radii
Luminosity = 1 Solar Luminosities

M Stars

M8 Main Sequence Star
Mass = 0.1 Solar Masses
Temperature = 2,500 K
Radii = 0.15 Solar Radii
Luminosity = 0.0008 Solar Luminosities

Habitable Zones

The Sun's "Habitable Zone"

Range of distances from the Sun where liquid water can be stable.
a=(0.84-1.7)AU --> Optimistic
a=(0.95-1.4)AU --> Conservative

Equilibrium Temperature

Worked Example A0 Star Luminosity = 63 times the luminosity of the Sun Scale from the Sun's habitable zone Inner edge = 6.72 AU Outer edge = 13.6 AU Width= 6.88 AU

Worked Example M8 Star Luminosity = Luminosity = 0.008 Sun Inner edge = 0.075 AU Outer edge = 0.152 AU Width= 0.077 AU

Habitable Zone is wider and further away for more massive stars


Why do stars shine?

Stars shine because they are hot.
--Emit light with a roughly thermal (blackbody) spectrum
--Internal heat "leaks" out of their surfaces.

Luminosity = rate of energy loss

To stay hot, stars must make up for the lost energy, otherwise they would cool and eventually fade out.

Generate energy by "burning" H into He in its core.

Fusion Energy

Fuse 1 gram of Hydrogen into 0.993 grams of Helium.
Leftover 0.007 grams is converted into energy

Hydrogen Fusion

Equivalence of mass and energy

-- Four hydrogen nuclei fuse into one helium nucleus, and the remaining 0.7% of the mass is converted to energy.

Stars overall lose a very small amount of mass (only about 0.07%).

Case Study: The Sun

How long can the Sun shine?

Need two numbers:
--How much internal heat there is in the Sun.
--How fast this heat is lost (Luminosity).

The Nuclear Lifetime

The nuclear lifetime is
f = fraction of nuclear fuel available for fusion
epsilon = efficiency of matter-energy conversion
M = mass; L = luminosity
For the Sun:
lifetime = 10 Gyr if f=10% of the Sun's H burned into He with epsilon=0.7% efficiency

Lifetime as a function of Mass

Examples: Sun: M = 1 Msun, ?MS ? 10 Gyr Massive Star (10 Msun) Lifetime ~ 10 Gyr/1000 ~ 10 Million Years Low-mass Star (0.1 Msun) Lifetime ~ 10 Gyr / 0.001 ~ 10 Trillion Years


If you see an O or B dwarf star, it must be young as they die after only a few Million years.

M dwarfs live a long time and age slowly.

The Sun is ~ 5 Billion years old, so it will last only for ~ 5 Billion years longer.

Definition: Stars which generate energy by fusion of H into He

The Main Sequence is a Mass Sequence

The location of a star along the M-S is determined by its Mass.
--Low-Mass Stars: Cool & Low Luminosity
--High-Mass Stars: Hot & High Luminosity
Result of the Mass-Luminosity Relation:

Planet Formation Timescale

Timescale for planet formation is very uncertain
But probably at least ~10 million years

Late Bombardment on Earth lasted for ~500 million years

Stellar lifetimes of at least 500 million years.

Lifetime Constraint

Ages greater than 500 million years

--> Masses less than ~3 Solar Masses

Excludes O and B type stars.

Brighter with Age?

M-S stars get slowly brighter with age.
Small effect: the sun has gotten 30% brighter in 4.5 Gyr

High mass stars get brighter faster

Low-mass stars have more stable, long-lived habitable zones.

Other Considerations

Tidal Locking: Bad for Habitability?
Closer planets may be tidally locked

One side of the planet gets all the radiation

Too Hot Side/Too Cold Side

Requires atmosphere to redistribute heat

Which stars have tidally locked planets in the habitable zone?

Habitable Zones orbiting M dwarfs are tidally locked!

Flares and UV radiation?

M stars have flares
--Flares are enormous outbursts of high-energy (X-ray and UV) radiation
--Could potentially be sterilizing
--On the contrary, may stimulate evolution

High mass stars emit lots of UV radiation
--Similarly could be good or bad for life.

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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