Red Stars, Blue Stars, Old Stars, New Stars Session 3

Julie Lutz

University of Washington

So We’ve Covered

• Basic physical parameters of stars

• Star clusters

• Interstellar medium

• How stars form and land on the main sequence

• Energy source on main sequence is H to He fusion.

What About the Extremes of Mass in Star Formation?

• Most massive stars observed are about 150 solar masses.

• Very rare!• Beyond that mass,

hard for star to hold itself together for long

• Internal energy trumps gravity

Lower Limit for Thermonuclear Fusion ~ 0.08 Solar Mass

• What happens if the mass of a forming object is less than that?

• It can still coalesce under forces of self-gravity, magnetic fields, etc.

Brown Dwarf Stars

• Show up at infrared wavelengths

• No thermonuclear reactions

• Fully gaseous and convective throughout

• Energy source is gravitational contraction

Masses of Brown Dwarfs

• From about 75-80 times the mass of Jupiter

• To about 15-20 times the mass of Jupiter

Low Surface Temperatures

• Visible in infrared• Molecules in

atmosphere (methane, ammonia, water, etc)

• Surface temperatures about 2000-500 K

First Brown Dwarf Discovered in 1995…Many Since Then

• If brown dwarfs exist, then maybe planets around other stars could exist, too.

• Had been hunting for extrasolar planets since 1940s without success until….

First Extrasolar Planet Discovered in 1995

• Tiny shifts in spectral lines due to planet influencing its star due to gravity

• See star’s spectrum shift periodically--can’t see planet directly

51 Pegasi

• Star that is very much like the sun in temperature and size

• Planet has a 4 hour orbit around star

• Yes, that’s VERY close

Artist’s Concept of 51 Cyg Star and Planet

Extrasolar Planets: Strategies and Methods for Searching

Planet-Hunting Strategies

• Look at stars like the sun first (particularly if your hunting technique requires observing one star at a time).

• If you can analyze many stars at one go, look at them all!

Doppler Shifts in Star’s Spectrum

• Small effect, need large telescope to detect.

• Ground-based• Many discoveries.

Transits

• Planet orbit oriented so it comes in front of its parent star periodically

• Causes a TINY dip in brightness because star is enormously brighter than planet

Space Missions

Kepler Mission--March 6, 2009

Kepler’s cameras take images of the same field every few seconds

Detecting a Transiting Planet with a Ground-Based Telescope

• Can be done, but requires a large telescope and a lot of images.

• First discovery announced in 2009

Direct Imaging

• Very difficult because star is so bright.• Best in infrared• Must block out as much starlight as possible

Ground-based Discoveries Come from Largest Telescopes

• Keck 10-m telescopes (Mauna Kea)

• Gemini 8-m telescopes (Mauna Kea and Chile)

• Subaru (Mauna Kea)• VLT (Chile)

About 500 Discovered So Far

• One star is now known to have 5 planets

• As techniques get better, expect MANY more discoveries

• Both ground-based and space observatories

Hope Eventually to Discover Earth-sized Planets

Recent Claim of an Earth-like Planet (Oct 2010)

• Gleise 581g• Orbiting a red main

sequence star• One of 7 planets• In the “Goldilocks”

zone (liquid water)• DISCOVERY

UNDER DISPUTE!

The Future

• Likely 1000s will be discovered by various techniques

• Will start getting an idea of how common they are, what kinds of stars have planets, how many planets around a star

• Start learning a bit about the planets besides their masses and orbital periods

Extrasolar Planetary Atmospheres

• Sodium• Water• Methane• Wind velocities in atmosphere, densities, etc

What Happens to Stars After the Main Sequence?

• Eventually all the hydrogen will be converted to helium in the star’s core.

• The star will lack an energy source to counterbalance the inward push of gravity.

Let’s Consider How Long It Will Take to Convert H to He in Core

• Thermonuclear reaction rate for H to He fusion depends on mass, density, temperature

• More massive stars do it much faster than less massive

• Calculate

Results

• A 1 solar mass star will stay on the main sequence about 12 billion years.

• Sun age 5 billion years• 120 solar mass-

20,000years• .08 solar mass--35

billion years

What Happens After Main Sequence?

• Star’s core collapses; outer layers respond by expanding and cooling.

• Star becomes a giant or a supergiant (depends on mass)

• Size of 10s to 100s times main sequence

What Happens to the Sun?

• In about 7 billion years the sun will become a giant star and will swell to roughly 30x its present size, engulfing Mercury and Venus

• This will take only about 50,000 years.

The Sun as a Giant Star

• While the outer part of the sun is expanding, the interior is heating up and eventually gets hot enough to fuse helium atoms into carbon atoms

• Hydrogen to helium fusion in a shell around core

H-R Diagram, 1 Msun

Stellar Middle Age

• Stars no longer hang out on the Main Sequence

• They move around in the giant and supergiant regions--patterns and timescales depend on mass

Fifteen Solar Masses

Antares

• Red supergiant• 15x mass of sun• 700x diameter of sun• Picture shows both the

star and the mass that it is losing in the form of gas and dust.

Antares is in Constellation of Scorpius the Scorpion

• Star name means “Rival of Mars”.

• Mars is also red.• Scorpion that stung

and killed Orion• Maui’s Hook

Giant/Supergiant Stages

• While the He is being converted to C in the core, there is a zone of H to He fusion surrounding the core

• When the core is all C, further changes occur and C to O fusion starts (with zones of He to C and H to He surrounding)

• Stars get an “onion” structure

The Outer Layers Change

• In part a response to what’s going on in the interior.

• At some stages stars can pulsate on timescales of days.

• They constantly lose mass from outer layers.

• We can follow these changes by calculating evolutionary tracks.

Some Comments on Stellar Middle Age

• The Sun (and other stars less than about 10 solar masses) will never be a supergiant.

• Stars more massive than about 10 solar masses do get to be supergiants.

• The massive stars fuse elements up to iron and they do it fast…timescales 1000s of years.

For Example, Cepheid Variables

• Named after delta Cephei, 4th brightest star in Cepheus.

• Varies by 0.7 mag with a period of 4.2 days.

• Star (a yellow giant) is pulsating.

Mira-A Red Giant That Pulsates

Mira Is in a Binary System

And It’s Moving 290,000 mi/hr Losing Mass

Even Red Dwarfs Have Interesting Things Going On

• They are by far the most common kind of star.

• Still on MS--slow evolution

• Have major flares• Planets

One Future Project

• Large Synoptic Survey Telescope• Start 2015, located in Chile• Will survey entire visible sky every 3 nights• UW is a major partner!

Conclusions

• About 500 stars are known to have one or more planets; many more discoveries ahead.

• Stellar evolution rates depend on mass. More massive=faster

• Stars move off the main sequence in response to changes in energy source and become giants or supergiants.