Light: the messenger from the stars
Stars are too far to visit. All we get is their light and other electromagnetic (EM) radiation: radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays. All of them travel at the speed of light, about 300 000 km/s. Different telescopes catch different kinds: radio dishes, infrared and X-ray space telescopes.
Spectra
A prism or grating splits light into a spectrum. A star's spectrum has dark absorption lines. Each element (hydrogen, helium, iron…) makes lines at its own wavelengths, like a barcode. So spectra tell us a star's composition.
Colour and temperature
Hotter objects glow bluer. The peak wavelength follows Wien's law: λmax = 2.9 × 10−3 m·K ÷ T. Stars are sorted by temperature into classes O B A F G K M (O hottest, about 30 000 K or more; M coolest, about 3 000 K). The Sun is a G star at about 5 800 K.
Doppler shift
If a star or galaxy moves away, its lines shift towards red (redshift); moving towards us, towards blue (blueshift).
Properties of stars and the H-R diagram
Key properties: temperature (from colour/spectrum), luminosity (true power output), apparent brightness (how bright it looks from Earth), size, mass and composition. A star can look dim because it is far away, not because it is weak.
The Hertzsprung–Russell (H-R) diagram plots luminosity (up) against temperature (hot on the left).
- Main sequence: a diagonal band where about 90% of stars are. They fuse hydrogen into helium in their cores. Heavier ones are hotter and brighter (top left).
- Giants and supergiants: top right. Cool but very bright, so they must be huge.
- White dwarfs: bottom left. Hot but dim, so they must be tiny (Earth-sized).
Our Sun is a middle main-sequence star. Its energy comes from nuclear fusion. It gives Earth light and heat, drives weather and the water cycle, powers photosynthesis, and its solar wind makes the auroras.
Birth, life and death of stars
- Nebula: a cloud of gas and dust. Gravity pulls a clump together.
- Protostar: the clump heats up as it shrinks.
- Main-sequence star: the core reaches about 10 million K and hydrogen fusion starts. Outward push of energy balances inward pull of gravity. This is the longest stage.
- When core hydrogen runs out, the star swells.
The ending depends on mass
- Low/medium mass (up to about 8 Suns): red giant → outer layers drift off as a planetary nebula → hot core left as a white dwarf.
- High mass (more than about 8 Suns): red supergiant → core collapses → supernova explosion → neutron star, or a black hole if the leftover core is very heavy.
Supernovae spread heavy elements (like the iron in your blood and the gold in jewellery) into space, where new stars and planets form.
Galaxies and the scale of the universe
A galaxy is a huge system of stars, gas, dust and dark matter held together by gravity. There are hundreds of billions of them.
- Spiral: flat rotating disc, central bulge, spiral arms with gas and young blue stars. Some have a central bar (barred spirals). The Milky Way is a barred spiral.
- Elliptical: round to egg-shaped, mostly old red stars, very little gas, few new stars.
- Irregular: no clear shape, often lots of gas and new stars (for example the Magellanic Clouds).
Measuring huge distances
- Astronomical unit (AU): Earth–Sun distance, about 1.5 × 108 km.
- Light-year (ly): distance light travels in one year, about 9.46 × 1012 km. It is a distance, not a time.
- Parsec (pc): about 3.26 ly. Using parallax (a near star's small shift as Earth orbits), d (pc) = 1 ÷ p (arcseconds).
Scale ladder: Earth → solar system → nearest star (4.2 ly) → Milky Way (about 100 000 ly across) → Andromeda (2.5 million ly) → galaxy clusters → the observable universe. In the night sky we see stars, constellations, planets, the Moon's phases and the Milky Way band.
The Big Bang: theory and evidence
The Big Bang theory says the universe began about 13.8 billion years ago in a very hot, dense state and has been expanding and cooling ever since. It was not an explosion in space; space itself expands.
Evidence
- Redshift and Hubble's law: distant galaxies move away, faster when farther: v = H0 × d, with H0 ≈ 70 km/s per megaparsec.
- Cosmic microwave background (CMB): a faint microwave glow from every direction, the cooled leftover heat (about 2.7 K).
- Amounts of light elements: about 75% hydrogen and 25% helium by mass, as the theory predicts.
Milestones and technology
Moving from an Earth-centred to a Sun-centred model, Galileo's telescope, spectroscopy, Hubble's discovery that galaxies recede, the CMB detection in 1965, and space telescopes such as Hubble and James Webb all changed our picture. India's AstroSat observes in ultraviolet and X-rays at the same time. Technology from astronomy, such as CCD image sensors, is now in every phone camera.
Try it
Make a spectrum: hold an old CD under a white LED or a street lamp and look at the rainbow. Different lamps (LED, sodium street light, tube light) give different patterns of bright lines, just as different stars do. Then use the slider in the 3D: predict what happens to colour and life span when you make a star 10 times heavier, then check.
Key formulas and definitions
- Wien's law: λmax = 2.9 × 10⁻³ m·K ÷ T
- Light-year: 1 ly ≈ 9.46 × 10¹² km
- Parallax distance: d (pc) = 1 ÷ p (arcsec); 1 pc ≈ 3.26 ly
- Hubble's law: v = H₀ × d, H₀ ≈ 70 km/s/Mpc
- Doppler (slow speeds): Δλ ÷ λ = v ÷ c
- Star classes hot → cool: O B A F G K M
- Mass > about 8 Suns → supernova; less → white dwarf
Worked examples
1. A star's light peaks at 500 nm. Estimate its surface temperature.
T = 2.9 × 10⁻³ ÷ λmax = 2.9 × 10⁻³ ÷ (500 × 10⁻⁹) = 5 800 K. This is like the Sun (class G).
2. Proxima Centauri is 4.2 light-years away. How far is that in km?
4.2 × 9.46 × 10¹² km ≈ 3.97 × 10¹³ km, about 40 trillion km.
3. A star shows a parallax of 0.25 arcseconds. Find its distance in parsecs and light-years.
d = 1 ÷ 0.25 = 4 pc. In light-years: 4 × 3.26 ≈ 13 ly.
4. Light from the Sun takes how long to reach Earth (distance 1.5 × 10⁸ km, speed 3 × 10⁵ km/s)?
t = d ÷ v = 1.5 × 10⁸ ÷ 3 × 10⁵ = 500 s ≈ 8.3 minutes. We see the Sun as it was 8 minutes ago.
5. A hydrogen line normally at 656 nm is seen at 662.56 nm in a galaxy. How fast is it moving, and which way?
Δλ = 6.56 nm, so Δλ/λ = 0.01. v = 0.01 × 3 × 10⁵ km/s = 3 000 km/s. The line shifted to longer (redder) wavelength, so the galaxy moves away.
6. Using H₀ = 70 km/s/Mpc, how far is a galaxy receding at 7 000 km/s?
d = v ÷ H₀ = 7 000 ÷ 70 = 100 Mpc ≈ 326 million light-years.
Common mistakes
- Thinking a light-year is a unit of time. It is a distance: how far light goes in one year.
- Reading the H-R diagram temperature axis the wrong way. Hot stars are on the LEFT.
- Thinking every star ends as a black hole. Only very massive stars do; the Sun will become a white dwarf.
- Picturing the Big Bang as an explosion at one spot in empty space. Space itself has been expanding everywhere.