1. Birth of Stars & Discovery of Planets Outside the Solar System
3 August 2005
AST 2010: Chapter 20

2. Questions about Star Formation
Are new stars still forming, or did star formation cease a long time ago?
If new stars are still being created, where is this occurring?
Are planets a natural result of star formation, or is our solar system unique in the universe?
If there are planets around distant stars, how can we observe them?
3 August 2005
AST 2010: Chapter 20

3. Known Basics about Stars
Stable (main-sequence) stars, such as the Sun, maintain equilibrium by producing energy through nuclear fusion in their cores
The ability to generate energy by fusion defines a star
Each second in the Sun, about 600 million tons of hydrogen undergo fusion into helium, with about 4 million tons turning to energy in the process
This rate of hydrogen use implies that eventually the Sun (and all other stars) will run out of central fuel
Stellar masses range from 1/12 MSun to ~200 MSun
Low-mass stars are far more common than high-mass ones
For main-sequence stars, the most massive (spectral type O) are also the most luminous and have the highest surface-temperature, whereas the least massive (spectral type M or L) are the least luminous and the coolest
A galaxy of stars, such as the Milky Way, contains huge amounts of gas and dust, enough to make billions of stars like the Sun
3 August 2005
AST 2010: Chapter 20

4. Giant Molecular Clouds
Vast clouds of gas (and dust) dot the Milky Way Galaxy
A giant molecular cloud is
an enormous, dense cloud of gas
so cold (10 to 20 K) that atoms are bound into molecules
The masses of giant molecular clouds range from about 1,000 MSun to about 3 million MSun
Within the clouds are lumps, regions of high density and low temperature
These conditions are believed to be just what is required to make new stars
The combination of high density and low temperature makes it possible for gravity to overcome pressure
3 August 2005
AST 2010: Chapter 20

5. Pillars of high-density, cool dust and gas in the central regions of the Eagle nebula, or M The colors in this image (taken by the Hubble Space Telescope) have been reassigned to enhance the level of detail visible in the image Go to website

6. Dense Globules in Eagle Nebula
One of the pillars in M16 appears to have dense, round pockets at the tips of finger-like features protruding from it
These pockets have been termed evaporating gas globules (e.g.g.s)
They may harbor embryonic stars
Video: zooming in to e.g.g.s in M16
3 August 2005
AST 2010: Chapter 20

7. Evaporating Gas Globules
3 August 2005
AST 2010: Chapter 20

8. EGGs in M16
3 August 2005
AST 2010: Chapter 20

9. Understanding Early Stages of Star Formation
The early stages of star formation are still shrouded in mystery because they are almost impossible to observe directly for three reasons:
The dust-shrouded interiors of molecular clouds where stellar births are thought to take place cannot be observed with visible light, but only with infrared and radio telescopes
The timescale for the initial collapse is estimated to be very short astronomically (thousands of years), implying that stars undergoing the collapse process are relatively few
The collapse of a new star occurs in a region so small that in most cases it cannot be observed with sufficient resolution using existing techniques
The current understanding of how star formation occurs is the result of theoretical calculations combined with the limited observations available
This implies that the present picture of star formation may be changed, or even contradicted, by future observations
3 August 2005
AST 2010: Chapter 20

10. Stellar Birth
The first step in the process of creating a star is the formation of a dense core within a clump of gas and dust
The process of core formation is not yet fully understood, but gravity can be expected to play an important role
Gravity causes the core to accumulate additional matter from the surrounding cloud material
The turbulence created inside a clump tends to cause the core and its surrounding material to spin
When sufficiently massive material has accumulated, gravity causes the core to collapse rapidly, and its density increases greatly as a result
During the time a dense core is contracting to become a true star — but before the fusion of protons to produce helium begins — the object is called a protostar
When the protostar is still accreting material from the surrounding cloud, dust and gas envelope the protostar, making it observable only in the infrared
3 August 2005
AST 2010: Chapter 20

11. Observation of Protostars
Infrared detectors enable observation of possible protostars
Many stars appear to be forming in the Orion Nebula above and to the right of the Trapezium stars
They can only be seen in the infrared image
Images from the Hubble Space Telescope
Visible Infrared
3 August 2005
AST 2010: Chapter 20

12. Winds & Jets
Once almost all of the available material has been accreted and the protostar has reached nearly its final mass, it is called a T Tauri star
after one of the best studied members of this class of stars
Upon reaching this stage of its development, the protostar starts producing a powerful stellar wind, consisting mainly of protons and electrons streaming away from its surface at speeds of a few hundred kilometers per second
The wind tends to emerge more easily in the direction of the protostar’s poles
The disk of material around its equator blocks the wind in this direction
Consequently, two jets of outflowing material appear in opposite directions from the protostar poles
3 August 2005
AST 2010: Chapter 20

13. Protostar Jets
The jets can collide with the material around the protostar and produce regions that emit light
These glowing regions are called Herbig-Haro (HH) objects
They allow us to estimate the location of the hidden protostar

14. True Star Being Born
Eventually, the stellar wind sweeps away the obscuring envelope of gas and dust, leaving behind the protostar and its surrounding disk
The protostar still continues to undergo gravitational contraction
This generates heat inside it and slowly increases its interior temperature
3 August 2005
AST 2010: Chapter 20

15. Birth of True Star
If the protostar is sufficiently massive, its central temperature will continue to increase to about 10 million K when nuclear fusion of hydrogen into helium begins inside its core
At this stage, the (proto)star is said to have reached the main sequence
It is now more or less in (hydrostatic) equilibrium and generates energy mainly through nuclear fusion inside its core
Thus astronomers say that a (true) star is born when it can sustain itself through nuclear reactions
Stars devote an average of 90% of their lives on the main sequence
3 August 2005
AST 2010: Chapter 20

16. Time to Reach Main-Sequence Stage
The development of contracting protostars can be tracked on the H-R diagram
The time to reach the main sequence is
short for high-mass stars
as low as 10,000 years
long for low-mass stars
up to 100 million years
3 August 2005
AST 2010: Chapter 20

17. H-R Diagram: Analogy to Weight versus Height for People
3 August 2005
AST 2010: Chapter 20

18. Weight and Height Change as Age Increases (Marlon Brando)
3 August 2005
AST 2010: Chapter 20

19. Different Paths for Different Body Types (Woody Allen)
3 August 2005
AST 2010: Chapter 20

20. Evidence that Planets Form around Other Stars
It is very hard to see a planet orbiting another star
Planets around other stars may be detected indirectly
One way is to look for disks of material from which planets might be condensing
A big disk is more visible than a small planet
Look for the evolution of disks, evidence for clumping into planets
3 August 2005
AST 2010: Chapter 20

21. Disks around Protostars
Four disks observed around stars in the Orion Nebula
The red glow at the center of each disk is believed to be a young star, no more than a million years old
3 August 2005
AST 2010: Chapter 20

22. Dust Ring around a Young Star
A debris disk has been found around a star called HR 4796A
The star has been estimated to be young, about 10 million years old
If there are newly formed planets around the star, they will concentrate the dust particles in the disk into clumps and arcs
3 August 2005
AST 2010: Chapter 20

23. Disk around Epsilon Erdani
Evidence for a clumpy disk has been found around a nearby star named Epsilon Eridani
The star is surrounded by a donut-shaped ring of dust that contains some bright patches
The bright spots might be warmer dust trapped around a planet that formed inside the donut
Alternatively, the spots could be a concentration of dust brought together by the gravitational influence of a planet orbiting just inside the ring
3 August 2005
AST 2010: Chapter 20

24. Planets Beyond the Solar System: Search & Discovery
If we can’t directly observe planets, can we indirectly observe them?
Kepler’s and Newton’s laws apply
In a star-planet system, both the planet and the star orbit a common center of mass
The planet’s motion has an effect on the star’s motion
As a result, the star wobbles a bit
From the observed motion and period of the wobble, the mass of the unseen planet can be deduced using Kepler’s laws
It is a planet if its mass is less than 1/100 the Sun’s mass (or about 10 times Jupiter’s mass)
3 August 2005
AST 2010: Chapter 20

25. Doppler Method for Detecting Planets
The star slightly wobbles due to the motion of the unseen companion planet
3 August 2005
AST 2010: Chapter 20

26. Discovered Planets
To date, more than 150 “planets” have been found in other star systems
Systems of 2, 3, and possibly more planets have been seen
The masses of the planets are measured in Jupiter-masses
3 August 2005
AST 2010: Chapter 20

27. Some Properties of First 101 Extrasolar Planets Found
3 August 2005
AST 2010: Chapter 20

28. Explaining the Planets Seen
Now that we have a large sample of “planetary” systems, astronomers need to refine, perhaps significantly, their current models of planetary formation
Most of the extrasolar “planets” found are not at all like the ones in our own solar system
Many of the extrasolar planets are similar to Jupiter in mass, or more massive, and have highly eccentric orbits close to their stars
This is a big surprise and is difficult for the early models to explain
The very massive planets orbiting close to their stars are sometimes called hot Jupiters
There are other surprises …
The formation of planetary systems is more complex and chaotic than we thought
Intensive search continues …
3 August 2005
AST 2010: Chapter 20