1. Star Classification

2. Properties of Stars
Mass – The single most important property that determines other properties of the star.
Luminosity – The total amount of energy (light) that a star emits into space.
Temperature – surface temperature, closely related to the luminosity and color of the star.
Spectral type – closely related to the surface temperature
Size – together with temperature determine the luminosity

3. The Magnitude System
Apparent magnitude describes the relative brightness of objects as they appears in sky.
A difference of 5 magnitudes is equivalent to a factor of 100 difference in apparent brightness.
 1st magnitude star is 100 times brighter than a 6th magnitude star.
A difference of one magnitude is a factor of 2.51 difference in brightness.
The larger the magnitude, the fainter the object
Objects with negative magnitude appear brighter than objects with positive apparent magnitude.
Apparent magnitude mv of selected objects :
The brightest star in the in night time sky, Sirius, is mv = -1.4
The Sun: mv = -27
The full Moon is -13
Maximum brightness of
Venus: mv = -4.7
Mars: mv = -2.9
Jupiter: mv = -2.8
Large Magellantic Cloud: mv = 0.9
Andromeda galaxy: mv = 4.3
Faintest star visible to human eyes: mv = 6

4. The Absolute Magnitude
A star’s absolute magnitude Mv is the apparent magnitude it would have if it were at a distance of 10 parsecs (32.6 light-years) from Earth.
The Sun’s absolute magnitude is Mv = 4.8
Sirius: Mv = +1.4
Betelgeuse: Mv = -5.1
Apparent magnitude tells us nothing about the luminosity of the objects, but it tell us how difficult it is to see the objects in the sky.
Absolute magnitude, on the other hand, is directly related to the luminosity of the object. But it does not tell us how bright they appear in the sky.
Astronomical Distance

5. Spectral Type of Stars
Spectral type is closely related to temperature

6. Stellar Classification
Each star is born with a specific mass.
This mass is the main factor in determining the star’s brightness, temperature, expected lifetime, type of death, and spectra.
Stars are classified according to their spectra.

7. Astronomers realized that large numbers of stars exhibit a small number of distinct patterns in their spectral features.

8. Stellar Classification
Spectral lines are caused by different elements present in a star.
The overall spectrum is strongly related to the surface temperature of the star.
Stars are then categorized into classes distinguished by the strength of the spectral lines and their shape.

9. Stellar Classification
The 7 classes of stars are:
O B A F G K M
From O to M:
Stars decrease in temperature
Stars generally become less massive (smaller)
Stars are ranked within class using a scale from 0-9 (0 being the hottest)

10. O B A F G K M Oh Be A Fine Guy/Girl, Kiss Me!
One Bug Ate Five Green Killer Moths
These are two of many mnemonic devices students and astronomers use to remember the proper order of stellar classification; from the hottest blue stars (O-type) to the coldest red stars (M-type).

11. Astronomical Distance Units
Light-year:
The distance light travels (in vacuum) in one year.
one light-year is 10 trillion (1013) km
Parsec: parallax & arcsecond
One parsec: the distance to an object with a parallax angle of 1 arcsecond.
One parsec equals to 3.26 light-year.
kiloparsecs: 1,000 parsecs.
megaparsecs: 1,000,000 parsec.
Absolute Magnitude

12. The Solar Neighborhood
The 30 closest stars to the Sun

13. Determination of Distance
Stellar Parallax
Knowledge of the distance to the stars is crucial for our determination of the luminosity of stars…
Current technology allows us to determine the distance accurately to within a few hundred light-years.
Hipparcos mission (European Space Agency) measured the stellar parallax of roughly 100,000 stars with precision of a few milli-arcseconds. So, it can measure distance of star up to 1,000 light-years away…
Simulation of Stellar Parallax…

14. Distances
The apparent motion of an object from two vantage points is known as parallax
Astronomers use parallax like surveyors use the angles at the baseline
The distance to a star with parallax p is given by the following formula
d (distance) = 1/p
d is in parsecs
p is in seconds of arc
p is half of the stars total change in position
1 parsec = 3.26 light years

15. Concept Check

16. Absolute and apparent magnitude
Apparent magnitude is how bright a star appears to be due to how close or far away it is.
Absolute magnitude is how much light is actually given off by a star.

17. Star Temperature
Stars in the sky show tinges of different colors which reveal the stars’ temperatures and composition.
Blue: hottest
Yellow: medium temperature
Red: coolest
Temperature is related to the mass of the star
the greater the mass the hotter the star
2/22/2019

18. Star Luminosity/Brightness
The brightness of a star depends upon the distance and its luminosity.
Apparent Brightness/Magnitude: a star’s brightness as seen from Earth.
Absolute Brightness/Luminosity: the actual brightness
2/22/2019

19. Parallax Used to triangulate the distance of a nearby star
Measures the change in angle between the star of interest and it’s background. Measurements are taken from opposite sides of the earth’s orbit.