Astronomy 1020-H Stellar Astronomy Spring_2015 Day-33

Course Announcements 1 Dark night observing session left: Thurs. Apr. 16 Alternative exercise is posted. Reports are due Wed. Apr. 22 Solar Rotation Project due Mon. Apr. 27

Protostars  Protostars are large, cool, and luminous.  They will emit infrared light.  Infrared studies of molecular regions reveal protostars and their disks.

 The protostar continues to accrete more material.  It continues to shrink and radiate away energy, balancing pressure and gravity.  The interior temperature and pressure rise.

 The low temperature of dust means that it glows in the infrared.  100 K dust:  10 K dust: MATH TOOLS 15.1

 The protostar’s energy source is gravitational energy.  As it shrinks, temperature rises in the core.  Hydrogen fusion begins in the core: It becomes a main sequence star.

 The temperature in the core must be hot enough for fusion, 10 million K.  Very low-mass stars (< 0.08 M  ) never start hydrogen fusion.  These are called brown dwarfs.

Concept Quiz—Energy What is the source of energy for a protostar on the Hayashi track? A.hydrogen fusion B.bipolar jets C.gravitational contraction D.angular momentum

Evolutionary Tracks  An individual star follows an evolutionary track on the Hertzsprung-Russell diagram.  This is the path of the temperature and luminosity with time.

Evolutionary Tracks  Protostars get less luminous (for lower masses), smaller in radius, and hotter.  The star moves on the Hayashi track and arrives on the main sequence.

1,056  A lower-mass star like the Sun is more luminous as a protostar than as a main sequence star, even though it is cooler as a protostar.  This is due to its physical size (radius). MATH TOOLS 15.2

Concept Quiz—Evolutionary Tracks Once fusion begins, a star moves to the left on the H-R diagram. Its luminosity does not change, but its temperature rises. The star is A. expanding. B. contracting. C. staying at the same radius. D. can’t tell from the information given

The more massive the protostar, the more rapidly it evolves

Concept Quiz Evolutionary Tracks Once fusion begins, a star moves to the left on the H-R diagram. Its luminosity does not change, but its temperature rises. The star is: A. Expanding. B. Contracting. C. Staying at the same radius. D. Can’t tell from the information given.

 Many or all protostars have material leaving in a bipolar outflow of jets.  Infalling and outflowing gas can be very complex.

Bipolar Outflows  Powerful jets can collide with the interstellar medium to make Herbig-Haro (HH) objects.  These can eject much of the mass that would otherwise land on the star.

 Star formation can make star clusters.  These are gravitationally bound groups of stars.  Clusters are good laboratories for testing our ideas of star formation and evolution.

 Star formation may take millions of years.  Some stars are more massive; others less so.  Higher-mass stars take less time forming and evolving.

Star Clusters All stars are: - same age - same composition -same distance Only difference: -mass

 New investigative methods can reveal misunderstandings.  Astronomers did not realize the presence and effect of gas and dust on starlight until spectroscopy was developed and applied. PROCESS OF SCIENCE

 A brown dwarf is not a star, nor a planet, but is in between.  Classified as L, T, or Y (cooler than M stars).  Glow in the infrared due to internal heat from gravitational contraction.  Over 1,000 have been found since the mid- 1990s. CONNECTIONS 15.1

 Stars are constantly radiating energy.  The energy available from fusion is very large, but finite.  Eventually, the fusion sources change, then run out.

 The star’s luminosity, size, or temperature will change.  A star’s life depends on mass and composition.  Stars of different masses evolve differently.

 The rates and types of fusion depend on the star’s mass.  Generally, stars with M < 3 M  share many characteristics: low-mass stars.  Intermediate-mass stars: 3 M  < M < 8 M   High-mass stars: M > 8 M 

 Higher temperature and pressure means faster nuclear fusion.  We can figure out main-sequence lifetimes: lifetime = (energy available) / (rate used).

 More mass = more fuel available.  Rate energy used = luminosity.  More massive stars have much higher luminosity.  They use their fuel up more quickly and leave the MS faster.

 Estimates can be made of star lifetimes, based on mass.  The mass-luminosity relationship:  The lifetime of a star depends on the amount of fuel (M) and how quickly it is used (L).  Can use this to compare other stars to the Sun: MATH TOOLS 16.1

 Main-sequence stars fuse hydrogen to helium in their cores.  Eventually, much of the core H is converted to He.  A core of He ash is built up (does not fuse at this point).