1. Years of Classifying Stars
Letters were assigned to stars of different spectrum.
Stars were classified by the elements they were made of
Stars are now classified by how hot they are and arranged by class and temperature
blue
30,000 degrees +
helium
10 Lacerate
Blue & white
7,500 – degrees
Hydrogen and helium
Vega, Rigel, Sirius
white Yellow
6,000 – 7, 500 degrees
Hydrogen and heavier elements
Procyon
Yellow
5,000-6,000 degrees
Calcium and other metals
The sun & Capella
orange
3,500-5,000 degrees
Calcium and other molecules
Aldebaran
red
Less than 3, 500 degrees
molecules
Betelgeuse

2. Types of Stars Stars are classified by how hot they are
Brightest stars in the sky are called 1st magnitude
Dimmest are called 6th magnitude
Positive number stars are dim stars, and negative numbers represent brightest stars

3. Quasars
A very bright (luminous) star-like object that generates energy at a high rate.
Thought to be the most distant objects in the universe
Some are as massive as 10 billion of our Suns
Most powerful energy source in the universe

4. Blue Stars The hottest stars are blue stars such as Rigel, =30,000 ºC
Brightest star in our night sky is Sirius

5. Pulsar
A rapidly spinning neutron star that emits rapid pulses of radiation
Develops at the end of a stars life as a last gasp before it goes “supernova”

6. Types of Brightness Apparent Magnitude Absolute Magnitude
The brightness of a star as it is seen from Earth
Our sun is very close to us so it has an apparent magnitude of but only an absolute magnitude of +4.8 which makes it “apparently small”
Absolute Magnitude
The actual brightness of a star
The brightness that a star would have at a distance of 32.6 light years from Earth
Sun’s magnitude is +4.8

7. Main Sequence Stars Second and longest stage of a star’s life
Energy is generated in the core of the star as hydrogen fuses to helium
Size of the star remains constant as long as the core is full of hydrogen
Move up and to the right on the HR diagram

8. Red Giant & Super giant
A star that expands and cools once all of its hydrogen is gone
Center of the star shrinks and its atmosphere grows
Can be 10 times bigger than our sun as red giant or 100 times bigger as a super giant

9. White Dwarf
A small hot star that is the leftover center of an older star
Has no hydrogen left
Final stage of a star’s life
Can shine for billions of years before they cool completely

10. When Stars Get Old . . . Black hole Supernova
an object so massive and dense that even light cannot escape its gravity
Gas or dust that sink into black hole from a star form x-ray light which may indicate a black holes’ existence
Supernova
massive blue stars use their hydrogen quickly and may explode in a huge bright flash
Can be brighter than an entire galaxy for several days
A collapsed star can become a pulsar

11. Hertzsprung-Russell Diagram
A graph that shows the relationship between a star’s surface temperature and absolute magnitude
Shows how they are classified by brightness and temperature and how they change over time
Hot blue stars are on the left cool red to the right, bright stars on top and dim stars on the bottom

12. Other Terms
Spectrum – the band of color produced when white light passes through a prism
Parallax – an apparent shift in the position of an object when viewed from different locations
Main sequence – the location on the H-R diagram where most stars lie
Neutron star – a star that has collapsed under gravity to the point that the electrons and protons have smashed together to form neutrons