Download February 16

Physical properties
Discussion
How can we determine the distances to
stars that are closer than 1 kpc?
Discussion
How can we determine the mass of a star?
Discussion
I have two stars
HD 1378326 and HD 1177892
Both are B8 stars. What do they have in common?
Discussion
Describe the three ways astronomers have of
determining a stars surface temperature.
Discussion
How does the brightness of an object depend
on distance?
Another inverse square law
The apparent brightness of a star falls as the
square of its distance from you.
Luminosity
Luminosity is an object’s intrinsic brightness,
the total amount of energy given off in a
second.
This is different from an object’s apparent
brightness
Discussion
What are some things that change the
apparent brightness of a star, how bright that
star appears in the sky?
Absolute magnitude
Luminosity is the same as apparent
brightness, removing variance due to
distance differences.
Absolute magnitude is magnitude a star
would have if it were 10 parsecs away.
Apparent and absolute
magnitudes
Apparent magnitude measures brightness in
the sky.
Absolute magnitude measures luminosity.
Discussion
I have two light bulbs, one is 25 watts and
the other is 100 watts. If the 25-watt light
bulb is 1 mile away, how far would I need to
place the 100-watt light bulb to have the
same apparent brightness?
Luminosity and Distance
If we know the luminosity of a star or galaxy,
we can figure out how far away it is by
comparing it to its apparent brightness.
Discussion
But, what if there is a lot of dust between us and
the object we are observing. That would make
the object appear fainter and we would be misled
into thinking the object was much farther away
than it really is. How can astronomers determine
if dust is making things fainter?
Discussion
Assuming a star acts as a blackbody, how is
the surface temperature related to its
brightness?
Determining Luminosity
The intensity of the emitted light from a
blackbody depends on the temperature.
Intensity is energy emitted per unit surface
area. To get the luminosity of a star, we need
to know the star’s size, i.e. its radius.
The larger the star, the more luminous it will
be at a given temperature.
Discussion
How do you think astronomers can estimate
the sizes of stars?
Occultation
We can find the temperature of a star from its
color, use the temperature to calculate the
intensity, and if the star just happens to be an
eclipsing binary, use its radius to calculate its
luminosity. Comparing its luminosity to its
apparent brightness we can finally arrive at its
distance.
More distances
The key to determining the luminosity of a
blackbody is knowing its temperature and
surface area.
The sizes of stars can vary from 0.01 R to 1000
R, where R is the radius of the Sun.
Discussion
Which star has a higher temperature, given
that both are spectral type B8?
Discussion
If two stars have the same mass, determined by
each being a binary star and using Kepler’s laws,
but one has a radius that is 10 times bigger than
the other, what is the difference in their surface
gravities?
Discussion
In which star do you think the atoms suffer
more collisions, one with low surface gravity,
or one with high surface gravity? Explain.
Discussion
What do you think is the difference
between the spectral lines produced by
atoms that rarely collide and those that
suffer thousands of collisions per second?
Discussion
Which B8 star has the larger surface gravity?
If these stars have about the same mass,
which is bigger?
Luminosity class
With temperature (from spectral type) and size
(from width of spectral lines) we can calculate
the luminosity of stars. The bigger the star the
higher its luminosity.
The width of the spectral lines is divided into
luminosity classes; I being the narrowest, II
being less narrow, and V being the broadest.
The Sun
The Sun is a G2 V, or sometimes referred to as
a G2 dwarf star.
Summing up
The spectrum of a star gives that star’s
surface temperature by the relative strength
of the absorption lines of different
elements.
The broadness of the lines gives the star’s
surface gravity. If we know the mass, we
can calculate the radius and thus the
luminosity of the star.
Stellar spectroscopy
Summing up
Knowing the star’s luminosity (its intrinsic
brightness) and comparing it with the star’s
apparent brightness, we can use the inverse
square law to calculate its distance.
Spectroscopic parallax
H-R diagrams
A plot of stars’ spectral class, or
temperature, or color ratio, vs. the
luminosity.
Hipparcos stars
Discussion
In the H-R diagram, 90% of stars lie along a
band running from the upper left to the
lower right which we call the main
sequence. The most luminous stars are the
bluest. Why is that?
The Main Sequence
Main sequence stars are fusing hydrogen in
their cores.
1% of stars are giant or supergiants in which
core H fusion has ceased.
9% are white dwarf stars.
Discussion
Most of the bright stars in the sky are giant or
supergiant stars. How can this be if only 1% of
the stars are giants and supergiants?
Mass-Luminosity relation for
main sequence stars
Discussion
Why do you think higher mass stars are
more luminous?
Main sequence is also
a mass sequence,
hotter, more luminous
stars on the main
sequence are more
massive.
Discussion
How do we get the distance to a star
with 1 kpc of the Sun?
What can we do if it is farther away?
Distance
If the star is within 1 kpc, we can use geometric
parallax.
If not, we can get a spectrum, determine its
spectral class and compare it with its apparent
brightness to determine the distance. This
method is often referred to as spectroscopic
parallax.
Discussion
What are the three ways we have of
determining a star’s surface temperature?
Temperature
Measure the star’s color index using UBVIR
photometry and use Wien’s law for
blackbodies.
Or get the stars spectrum and use the relative
strength of its absorption lines for different
elements. That is, determine the star’s
spectral classification or fit the continuum
Discussion
How do we determine a star’s mass if it is in
a binary system?
What if it is not a binary, but is a main
sequence star?
Mass
If the star is in a binary system, can use Kepler’s
3rd law to calculate the mass.
If the star is a main sequence star you could use
the mass-luminosity relation.
If not, you need the star’s radius and the width
of the spectral lines to find the surface gravity.
Radius
If it is the right type of eclipsing binary,
measure the length of the eclipses.
If not, the radius of a star can be roughly
determined from its luminosity class (width of
spectral lines), apparent brightness and
distance.
Composition
Can be determined from the strength of the
absorption lines in its spectrum, after the
effects of temperature have been removed.
Discussion
How do we get the radial velocity of a star?
Radial Velocity
Radial velocity can be determined using the
Doppler effect or shifts in the absorption lines
in the spectra.
Proper motions
To measure a star’s motion perpendicular to
the line of sight, i.e. in the plane of the sky, we
must know its distance and wait to see it move.
Typically, proper motion is measured in
milliarcsec/year.
Barnard’s Star
Has the highest proper motion of about 10
arcsec/year
Stellar evolution
How can we ever know how stars will
evolve if they take millions and billions of
year to do so?
Discussion
Aliens visit Earth in order to determine the life
cycle of humans. But to avoid detection they
only gather 1 seconds worth of data. What
clues can they get and how from such a short
visit?
The main sequence
90% of stars are main sequence stars. Thus, if
star formation is relatively constant, stars must
spend about 90% of their lives on the main
sequence.
Pleiades
M 80
Orion nebula
Discussion
In the previous pictures, why does some of the
gas appear blue, some red and some dark?
Emission nebula
Hot stars ionize the hydrogen in the interstellar
gas. When the hydrogen recombines with an
electron, it emits Balmer photons, primarily in
the red in photographs.
Reflection nebula
Stars are not hot enough to ionize the hydrogen
gas or gas is too thin to recombine with
electrons. Instead the light we see is scattered
star light, and because shorter wavelengths are
more easily scatter, reflection nebulae appear
blue.
Dark nebula
Dust and gas that is thick enough to block light
from passing through it.
Discussion
How can astronomers tell that the dark lanes in
the previous picture are actually caused by
opaque gas and dust blocking the light from
behind it and not just dark empty space devoid
of stars and gas? What observations could you
make to prove it?
Discussion
Many open star clusters are embedded in a
cloud of interstellar gas and dust. What do you
think is going to happen to that gas and dust
over time?
NGC 3603
NGC 2264