Download A stars

Homework #10
due Friday, December 3, 5:00 pm
Homework #11
Will be posted along with answers.
Score will be equal to highest score on homeworks 1-10.
Life beyond the solar system
Star
A mass of gas held
together by gravity
in which the central
temperatures and
densities are
sufficient for steady
nuclear fusion
reactions to occur.
A star’s color is indicative of its temperature
Color
Spectral type
Temperature
Stars are often described by their “spectral type”,
which is a function of its temperature
The required mass to
have fusion reactions
in the core is at least a
few percent of the
mass of the sun.
Nuclear fusion occurs
in the core of a star.
Fusion of hydrogen to
helium is the nuclear
process functioning
over most of a star’s
lifetime.
We refer to this time as
the Main Sequence
lifetime
A convenient way to
gain insight into the
life and death of stars
is through the
“Hertzsprung-Russell
Diagram”
Hertzsprung-Russell Diagram
A plot of the temperature of stars against their brightness (luminosity)
Hertzsprung-Russell
Diagram
Stars do not fall
everywhere in this diagram
An HR diagram for about 15,000
stars within 100 parsecs (326
light years) of the Sun.
Most stars lie along the
“Main Sequence”
Hot stars (bluer) are
found at the upper left
hand end of the Main
Sequence while cooler
(redder) stars are found
to the lower right.
Stars are all classified
according to temperature and
spectral type, with the hotter
stars called ‘O’ type stars and
the coolest called ‘M’ type
stars. The order of
classification is:
O-B-A-F-G-K-M
Stars live most of
their lives on the
“Main Sequence”.
These stars
generate energy by
nuclear fusion of
hydrogen into
helium in their core.
Very
rare
Hotter “Main
Sequence”
stars are
much less
common than
cooler Main
Sequence
stars
Very
common
Hotter stars
have shorter
Main
Sequence
lifetimes than
cooler stars
107 yrs
108 yrs
109 yrs
1010 yrs
1011 yrs
A star “moves” on the HR
diagram as it ages
Collapse of
protostar to
Main Sequence
Moving up
Main
Sequence
Hydrogen begins
to run out in core.
Expansion to
giant
Depletion of fuel in core.
Shedding of mass
Collapse of remnant
- dead star
Major Factors for life on the
Surface of a Planet:
 Location, location, location:
– must lie within a star’s habitable zone
Major Factors for life on the
Surface of a Planet:
 Location, location, location:
– must lie within a star’s habitable zone
 Size is important:
– Large enough to retain an atmosphere substantial
enough for liquid water
– Large enough to retain internal heat and have plate
tectonics for climate stabilization
The Habitable Zone
An imaginary spherical shell surrounding a star
throughout which the surface temperatures of any
planets present might be conducive to the origin
and development of life as we know it.
Essentially a zone in which
conditions allow for liquid water
on the surface of a planet.
The Sun’s
Habitable
Zone
(today)
The Sun’s
Habitable
Zone
(thru time)
The Sun’s
brightness
(luminosity) has
changed with time.
Habitable Zones for Different Stars
Lower mass (cooler) stars
have smaller habitable zones
By contrast, the HZ of a highly luminous star would in principle be
very wide, its inner margin beginning perhaps several hundred
million km out and stretching to a distance of a billion km or more.
The size and location of the HZ depends on
the nature of the star
Hot, luminous stars – spectral types "earlier" than that of the Sun (G3-G9, F, A,
B, and O) – have wide HZs, the inner margins of which are located relatively
far out:
To enjoy terrestrial temperatures:
Around Sirius (Spectral type A1: 26 times more luminous than the Sun),
an Earth-sized planet would have to orbit at about the distance of Jupiter
from the star.
Around Epsilon Indi (Spectral type K5: about one-tenth the Sun's
luminosity), an Earth-sized planet would have to orbit at about the
distance of Mercury from the star.
The size and location of the HZ depends on
the nature of the star
The situation becomes even more extreme in the case of a red dwarf, such as
Barnard's Star (M4: about 2,000 times less luminous than the Sun), the HZ of
which would extend only between about 750,000 and 2 million km (0.02 to
0.06 AU).
However: if planets exist too close to its parent star, the development of
life might be made problematic because the tidal friction would have led to
synchronous rotation.
 The same side of the planet will always face the star.
More massive, brighter stars have wider HZ.
However, massive, bright stars are much more short-lived than smaller, stars.
In the case of the massive O stars and B main sequence stars, these very
objects race through their life-cycles in only a few tens of millions of years
– too quickly to allow even primitive life-forms to emerge.
Less massive, cooler stars have narrower HZ.
But these stars live much longer than larger, more massive stars.
In the case of the low mass K and M main sequence stars, these very
objects live many tens to hundreds of billions of years – considerable time
to allow even advanced life-forms to emerge.
SO, WHERE TO SEARCH?
LIFE? Given the rate of
evolution of life on Earth, it is
possible that microorganisms
might have time to develop on
worlds around A stars.
INTELLIGENT LIFE? But in the
search for extraterrestrial
intelligence, the HZs around F
stars and later must be
considered the most likely
places to look.
There are 200 billion stars in our galaxy…
…one of them is our Sun.
2
The sun has eight planets…
…we know of one that has life.
Are there other planets in the universe?
Is there another Earth out there?
2
Thousands of years ago, Greek philosophers
speculated.
“There are infinite
worlds both like and
unlike this world of
ours...We must believe
that in all worlds there
are living creatures and
planets and other things
we see in this world.”
Epicurius
c. 300 B.C
And so did medieval
scholars.
The year 1584
"There are countless suns and
countless earths all rotating
around their suns in exactly
the same way as the seven
planets of our system . . . The
countless worlds in the
universe are no worse and no
less inhabited than our Earth”
Giordano Bruno
in De L'infinito
Universo E Mondi
4
1995
Discovery of the first planet around another star.
Didier Queloz and Michel Mayor
A Swiss team discovers a planet – 51 Pegasi –
48 light years from Earth.
Artist's concept of an extrasolar planet (Greg Bacon, STScI)
7
And then the discoveries started rolling in:
“New Planet Seen Outside Solar System”
New York Times
April 19, 1996
“10 More Planets Discovered”
Washington Post
August 6, 2000
“First new solar system discovered”
USA TODAY
April 16, 1999
A useful site to keep current on discoveries:
http://planetquest.jpl.nasa.gov/