Introduction to Astrophysics
Explore the Hertzsprung-Russell diagram, trace the life cycles of stars, understand black holes, and discover how redshift reveals an expanding universe.
Pax says: "Every atom in your body was forged inside a star. Let's explore the incredible life stories of these cosmic furnaces and how they shape the universe!"
The Hertzsprung-Russell Diagram
The Hertzsprung-Russell (HR) diagram is one of the most important tools in astrophysics. It plots stars by their luminosity (brightness, vertical axis) against their surface temperature (horizontal axis, with hotter stars on the left). Stars are not scattered randomly -- they cluster into distinct groups that reveal their evolutionary stage.
Simplified HR Diagram
Luminosity →
← Surface Temperature (hot to cool)
Main Sequence
Red
Giants
Supergiants
White
Dwarfs
Sun
Main Sequence Stars
~90% of all stars lie on the main sequence, a diagonal band from hot/bright (upper left) to cool/dim (lower right). These stars fuse hydrogen into helium in their cores. Our Sun is a main sequence star.
Off the Main Sequence
Red giants/supergiants (upper right): cool but very luminous due to enormous size. White dwarfs (lower left): very hot but dim because they are tiny remnants of dead stars.
Stellar Evolution
Stars are born, live, and die over millions to billions of years. The mass of a star at birth determines its entire life story -- how long it lives, how brightly it shines, and how it ultimately dies. Massive stars burn hot and fast, while smaller stars burn slowly and steadily.
Life Cycle of Stars
Nebula
Cloud of gas and dust collapses under gravity
Main Sequence Star
Hydrogen fusion in core (our Sun: ~10 billion years)
Low/Medium Mass
Red Giant
White Dwarf
High Mass
Supergiant
Supernova
Neutron Star / Black Hole
Key Insight: A star's mass determines its fate. Stars less than about 8 solar masses end as white dwarfs. Stars above this threshold explode as supernovae, leaving behind neutron stars or, for the most massive, black holes -- regions where gravity is so intense that not even light can escape.
Redshift and the Expanding Universe
When a light source moves away from an observer, its light waves are stretched to longer wavelengths -- a phenomenon called redshift. In 1929, Edwin Hubble discovered that nearly all distant galaxies show redshift, and the farther away they are, the greater the redshift. This means the universe is expanding.
Doppler Effect for Light
Hubble's Law
v = H0d
where v = recession velocity of galaxy, H0 = Hubble constant (~70 km s-1 Mpc-1), d = distance to galaxy
This relationship shows that more distant galaxies recede faster, confirming the expansion of space itself.
Evidence for the Big Bang
The expansion of the universe, combined with the cosmic microwave background radiation (residual heat from the early universe) and the observed abundance of light elements (hydrogen and helium), provides strong evidence that the universe began from an extremely hot, dense state -- the Big Bang -- approximately 13.8 billion years ago.
Key Vocabulary
HR Diagram
A scatter plot of stars showing luminosity versus surface temperature. Stars cluster into groups (main sequence, giants, white dwarfs) that reveal their evolutionary stage.
Redshift
The increase in wavelength (shift toward red) of light from objects moving away from the observer, caused by the Doppler effect or the expansion of space.
Black Hole
A region of spacetime where gravity is so extreme that nothing, including light, can escape. Formed from the collapse of the most massive stars after a supernova.
Supernova
A catastrophic explosion of a massive star at the end of its life, briefly outshining an entire galaxy and dispersing heavy elements into space for future star and planet formation.
Worked Examples
A galaxy has a recession velocity of 2100 km s-1. Using Hubble's constant H0 = 70 km s-1 Mpc-1, calculate its distance.
Step 1: Use Hubble's Law: v = H0d, so d = v / H0
Step 2: d = 2100 / 70 = 30 Mpc (megaparsecs)
Answer: The galaxy is approximately 30 Mpc (about 97.8 million light-years) away.
A star on the HR diagram is located in the upper-right region. It has a surface temperature of about 3500 K and luminosity 1000 times the Sun's. Classify this star.
Step 1: Upper-right on the HR diagram means low temperature but high luminosity.
Step 2: Low temperature (3500 K) gives a reddish colour. High luminosity despite low temperature means the star must have a very large radius.
Answer: This star is a red giant. Its enormous size compensates for its low surface temperature, giving it high overall luminosity.
A hydrogen spectral line normally observed at 486 nm is measured at 491 nm from a distant galaxy. Calculate the redshift z and estimate the galaxy's recession velocity.
Step 1: Redshift z = Δλ / λ0 = (491 - 486) / 486 = 5 / 486 = 0.0103
Step 2: For small z: v ≈ zc = 0.0103 × 3 × 105 km s-1
Answer: v ≈ 3090 km s-1. The galaxy is receding at about 3090 km/s due to the expansion of the universe.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
On the HR diagram, the main sequence runs from:
Question 2
The primary factor that determines the life cycle and ultimate fate of a star is its:
Question 3
Redshift of light from distant galaxies provides evidence that:
Question 4
A star like our Sun will eventually end its life as a:
Question 5
According to Hubble's Law, a galaxy twice as far away as another will have a recession velocity that is:
Key Concepts Summary
- ●The HR diagram plots luminosity vs. temperature and reveals star groups: main sequence, red giants/supergiants, and white dwarfs.
- ●A star's initial mass determines its lifespan and fate: low-mass stars become white dwarfs; massive stars end in supernovae, leaving neutron stars or black holes.
- ●Redshift of distant galaxies shows they are receding, providing key evidence for the expansion of the universe.
- ●Hubble's Law (v = H0d) describes the linear relationship between a galaxy's distance and its recession velocity.
- ●The expansion of the universe, cosmic microwave background, and light element abundances together support the Big Bang model of the origin of the universe.