Download Apparent brightness

Unit 4 – Measuring Stars
 Create a Unit cover page for unit 4. (You will copy all
of the unit 4 objectives on this page)
 Create a Unit 4 WWK page
 Create a Unit 4 Must do page
Objectives (write on cover page)
 Explain how stellar distances are determined.
 Explain how physical laws are used to estimate stellar sizes.
 Distinguish between luminosity and brightness and explain
how stellar luminosity is determined.
 Explain how stars are classified according to their colors,
surface temperatures and spectral characteristics, and tell why
such a classification is useful.
 State how an H-R diagram is constructed, and summarize the
properties of the different types of stars that such a diagram
helps us to identify.
WWK
 Stellar Parallax – the apparent motion of a relatively
close object with respect to a more distant background
as the location of the observer changes.
 Parsec – distance at which a star must lie in order for
its measured parallax to be exactly 1 arc second. 1
parsec equals 206,000 A.U.
 Parallactic angle is
the parallax
 Measured in
arcseconds.
 1° = 3600 arcseconds
Must Do
 Please write a
statement about
what this
diagram is trying
to express.
 Be ready to
discuss.
WWK
 Giants – star with a radius between 10 and 100 times that of the
Sun
 Supergiants – A star with a radius between 100 and 1000 times
that of the Sun
 Red giant – A giant star whose surface temperature is relatively
low, so that it glows red.
 Dwarf – Any star with radius comparable to, or smaller than, that
of the Sun (including the Sun itself)
 White dwarf – A dwarf star with sufficiently high surface
temperature that it glows white
Stellar Sizes
 Measured directly - using geometry and knowing how far
away the star is. Astronomers have measured a few dozen
stars this way.
l
 Most stars are too far awayr µfor
this to work
T
2
 Measured indirectly – luminosity and temperature gives us
an indirect measurement as to the size of the star.
 Luminosity is proportional to the radius squared times the
temperature to the fourth power.
rµ
l
T2
 Called the radius-luminosity-temperature relationship.
SWK – Sirius (dog star)
 Radius and luminosity

Sirius is a two star system 8.6 light years from
Earth.

It consists of the main sequence star Sirius A
and its small white dwarf companion Sirius B.

White dwarfs are the core remains of stars that
have exhausted their fuel and shed their outer
layers.

Sirius B is the closest white dwarf star to Earth.

The force of gravity on Sirius B is 350,000
stronger than on Earth, meaning 3 grams of
matter (roughly a sugar cube) would weigh
1,000 kilos (2,200 pounds)!

Sirius is the brightest star in the night sky and
the nearest that can be seen without the aid of
a telescope.
 Ar=71% larger than our Sun
 Al = 25 x the sun
 Br = smaller than earth
(but more dense)
 Bl = 3% the sun
Luminosity and Apparent
Brightness
 Absolute brightness – An intrinsic property,
luminosity
 Apparent brightness – How much energy is
striking a light detector per unit of time
 Energy produced by the star as seen from earth
 Uses the magnitude scale -
Ancient Magnitude Scale
 2nd century BC astronomer Hipparchus (6 groups)
 Brighter stars were ranked first magnitude and fainter
stars were classified 6th magnitude.
 1-6 classification spans a factor of 100 in apparent
brightness.
 Each magnitude is a difference of 2.5 in apparent
brightness. 1st magnitude stars are 2.5 times brighter
than 2nd magnitude stars.
Modern Magnitude Scale
 A change in 5 magnitude represents a factor of 100 in
apparent brightness.
 Now called apparent magnitudes
 No longer limited to whole numbers
 4 - 4.5 - 5
 1-6 barely even covers it
 Hubble can see 30 magnitude stars
 The sun is a -26.8 magnitude star
Mathematical Relationship
 Knowing a stars apparent magnitude and
its distance allows us to compute its
absolute magnitude
 Alternatively, knowing a star’s apparent
and absolute magnitude allows us to
determine its distance.
Temperature and Color
 Color index – ratio of its B (blue) to V (visible)
intensities
Classification of Stars
 Color and temperature can classify stars well enough but
SPECTROSCOPY gives us spectral-line radiation which
is a much more detailed classification theme.
 The composition of
these stars are the same
the difference in
absorption spectra is
temperature.
 Why do the hotter stars
have less absorption
lines?
Detailed Spectra
 20,000 K and up show strong ionized Helium because…
 it takes a lot of energy to excite tightly bound atoms.
 Hydrogen is very weak in these hot stars because…
 It is ionized so few hydrogen atoms have electrons
 In cooler stars absorption lines are caused by molecules that
are still able to maintain their bonds.
Spectral Classification
 Between 1880 and 1920 stars were classified by their
spectral analysis even before they knew how atoms
worked.
 They were categorized by the intensity of the hydrogen
lines in a A, B, C, D, E…P
 Then it was realized that they could be organized by
temperature so the new schema was born.
 “Oh Be A Fine Guy, Kiss Me.”
Our Sun is a G2 Star
(each letter is split into 10 subdivisions)