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

Spectral typeTemperature

Color

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.

Hotter “Main Sequence” stars are much less common than cooler Main Sequence stars

Very rare

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.

2

There are 200 billion stars in our galaxy…

…one of them is our Sun.

The sun has eight planets…

…we know of one that has life.

2

Is there another Earth out there?

Are there other planets in the universe?

“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

Thousands of years ago, Greek philosophers speculated.

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 Brunoin De L'infinito Universo E Mondi 4

1995Discovery of the first planet around another star.

A Swiss team discovers a planet – 51 Pegasi –48 light years from Earth.

Artist's concept of an extrasolar planet (Greg Bacon, STScI)7

Didier Queloz and Michel Mayor

And then the discoveries started rolling in:

“First new solar system discovered”USA TODAY

April 16, 1999

“10 More Planets Discovered” Washington Post

August 6, 2000

“New Planet Seen Outside Solar System”New York TimesApril 19, 1996

http://planetquest.jpl.nasa.gov/A useful site to keep current on discoveries: