Galaxies, the light year and telescopes
Stars look like dots at night, yet telescopes tell us there is a vast universe of billions of stars behind them.
A collection of billions of stars, their planetary systems and gas clouds is a galaxy (दीर्घिका). Our galaxy, the Milky Way (आकाशगंगा), is spiral in shape. The distance light travels in one year is a light year (प्रकाशवर्ष), about 9.5 × 10¹² km.
Rule 1: Types of galaxies — By shape there are three kinds of galaxies: spiral, elliptical and irregular.
- Easy : Our Milky Way is a spiral galaxy.
- Medium : A galaxy with no definite shape is irregular.
- Exam-type : The Milky Way is like a disc with a bulge in the middle; the solar system lies far from its centre.
Rule 2: Working with the light year — Light moves at about 3,00,000 km/s. Seconds in a year × speed = one light year. It is a unit of distance, not of time.
- Easy : Sunlight reaches us in about 8 minutes.
- Medium : Moonlight reaches us in about 1 s, so the Moon is near.
- Exam-type : Alpha Centauri, the nearest star, is 4.2 light years away; the light we see left it 4.2 years ago.
Rule 3: Why telescopes? — Our eyes see only a few thousand stars. Telescopes let us study distant objects. Telescopes on satellites observe from above the atmosphere, so their pictures are clearer.
- Easy : About 4000 stars are visible to the naked eye at night.
- Medium : The Hubble telescope orbits outside the atmosphere.
- Exam-type : Dust and air blur images in ground telescopes, so satellite telescopes do better.
The light year is a unit of distance.
Thinking a light year is a time. It is a distance.
Memory clue : "Light year = one year of light travel" — the name has "year", yet it is a distance.
Spot the difference
| Galaxy | Shape | Example |
|---|---|---|
| Spiral | Curved arms | Milky Way |
| Elliptical | Oval | Some large galaxies |
| Irregular | No fixed shape | Some small galaxies |
Practice questions
- 1What is our galaxy called?
- 2The light year is a unit of what?
- 3Name three types of galaxies.
- 4How long does sunlight take to reach us?
- 5What is the speed of light?
- 6Why are satellite telescopes better?
- 7What do we see when we look at Alpha Centauri today?
- 8Why is "light year = time" wrong?
Show answers (try them yourself first)
- 1The Milky Way.
- 2Distance.
- 3Spiral, elliptical, irregular.
- 4About 8 minutes.
- 5About 3,00,000 km/s.
- 6They observe from above the atmosphere.
- 7Light that left 4.2 years ago.
- 8It is the distance covered by light.
The Sun: an ordinary star
The Sun looks far bigger than other stars only because it is near; in truth it is smaller than many and larger than many.
Stars are huge spheres of hot gas. The Sun is about 72 % hydrogen, 26 % helium and 2 % heavier elements. Masses of stars are stated relative to the Sun, whose mass is used as the unit.
Rule 1: Properties of the Sun — The Sun has a mass of about 2 × 10³⁰ kg, a radius of 6,95,700 km, a surface temperature near 5800 K and a central temperature near 1.5 × 10⁷ K. Its age is about 4.5 billion years.
- Easy : The Sun's centre is far hotter than its surface.
- Medium : The Sun's mass is about 3.3 lakh times the Earth's.
- Exam-type : The Sun's radius is about 100 times the Earth's.
Rule 2: Comparing with other stars — Other stars have masses from about one-tenth to a hundred times the Sun's, and radii from one-tenth to a thousand times the Sun's.
- Easy : Our Sun is one ordinary star among billions.
- Medium : A supergiant is far larger than the Sun, and a white dwarf far smaller.
- Exam-type : A star of 20 solar masses has 20 times the Sun's mass.
The Sun's properties have not changed in the last 4.5 billion years.
The Sun is not the largest star; it is merely the nearest.
Memory clue : "72 – 26 – 2" — hydrogen, helium, heavier elements (per cent).
Spot the difference
| Property | Value for the Sun |
|---|---|
| Mass | about 2 × 10³⁰ kg |
| Surface temperature | about 5800 K |
| Central temperature | about 1.5 × 10⁷ K |
| Age | about 4.5 billion years |
Practice questions
- 1Which gas is most abundant in the Sun?
- 2What kind of star is the Sun?
- 3What is the Sun's age?
- 4Why does the Sun look larger than other stars?
- 5In what unit are stellar masses expressed?
- 6What per cent of the Sun is helium?
- 7What would a change in the Sun's properties have caused?
- 8Why is the Sun called ordinary?
Show answers (try them yourself first)
- 1Hydrogen (about 72 %).
- 2An ordinary star.
- 3About 4.5 billion years.
- 4Because it is nearest to us.
- 5In multiples of the Sun's mass.
- 6About 26 %.
- 7The Earth and life on it would have changed.
- 8Because billions of stars have lower or higher mass, size and temperature.
Birth of a star
The empty space between stars holds huge clouds of gas and dust, and new stars are born from them.
A cloud of gas and dust between stars is an interstellar cloud (आंतरतारकीय ढग). It contracts, so its density and temperature rise. When the centre is dense and hot enough, hydrogen nuclei fuse into helium, nuclear energy is released, the gas ball glows by itself and a star is born.
Rule 1: From cloud to star — Some disturbance makes the cloud contract. Gravitational potential energy turns into heat, so the temperature rises.
- Easy : An interstellar cloud is a few light years across.
- Medium : When temperature and density are high enough, fusion begins at the centre.
- Exam-type : Hydrogen is the fuel in fusion; it yields helium and energy.
Rule 2: Many stars from one cloud — One giant cloud can form thousands of stars, seen as a star cluster.
- Easy : Most stars in a cluster are born from one cloud.
- Medium : So the stars in a cluster are of similar age.
- Exam-type : Massive stars burn out sooner, so old clusters show fewer of them.
A star is born once fusion starts.
The star does not burn coal-like fuel; its energy comes from fusion of nuclei.
Memory clue : "Contract → heat → fuse → star" — order of birth.
Spot the difference
| Step | What happens |
|---|---|
| 1 | The interstellar cloud contracts |
| 2 | Density and temperature rise |
| 3 | Hydrogen fusion starts at the centre |
| 4 | The gas ball shines on its own: a star |
Practice questions
- 1Stars are born from what?
- 2What fuel does a star use?
- 3What happens to the temperature as a cloud contracts?
- 4In fusion, what forms from what?
- 5When does a star begin to shine?
- 6What is a star cluster?
- 7Why does contraction raise the temperature?
- 8What do cloud sizes of a few light years tell us?
Show answers (try them yourself first)
- 1Interstellar clouds.
- 2Hydrogen.
- 3It rises.
- 4Helium from hydrogen nuclei.
- 5When energy generation begins at the centre.
- 6A group of thousands of stars born from one cloud.
- 7Gravitational potential energy changes into heat.
- 8They are enormous; light itself takes years to cross.
Stability of a star
Hot gas spreads through a room, so why does the Sun's gas not spread into space? Gravity is the answer.
Gravity pulls the gas inward, and the pressure of hot gas pushes outward. When the two balance, the star is stable (स्थिर). If gravity wins the star contracts; if pressure wins it expands.
Rule 1: The tug-of-war picture — In tug-of-war equal pulls keep the rope's middle still. In a star, gravity and gas pressure are the two teams.
- Easy : In balance the star is stable.
- Medium : If gravity is stronger, the star contracts.
- Exam-type : With no gas pressure the Sun would collapse in 1–2 hours.
Rule 2: Energy and stability — Gas pressure depends on temperature and density. To keep the temperature steady, energy must keep being made at the centre.
- Easy : Higher temperature and density mean higher pressure.
- Medium : When fuel ends, energy generation stops and temperature falls.
- Exam-type : Pressure falls with temperature; gravity wins and the star contracts.
Stability of a star
In the app you can move this figure with your finger.
Open in the appGravity pulls in, gas pressure pushes out.
Stable does not mean unchanging; it means two forces balance.
Memory clue : "Gravity = Pressure → stable" — inward and outward forces equal.
Spot the difference
| Condition | Result |
|---|---|
| Gravity = pressure | Star is stable |
| Gravity > pressure | Star contracts |
| Pressure > gravity | Star expands |
Practice questions
- 1When is a star stable?
- 2In which direction does gravity act?
- 3In which direction does gas pressure act?
- 4What if gravity becomes stronger than pressure?
- 5On what does gas pressure depend?
- 6Why must a star keep generating energy?
- 7What happens when the fuel runs out?
- 8How does tug-of-war match a star?
Show answers (try them yourself first)
- 1When gravity and gas pressure balance.
- 2Towards the centre (inward).
- 3Outward, away from the centre.
- 4The star contracts.
- 5Temperature and density.
- 6To keep the temperature, hence the pressure.
- 7Temperature and pressure drop; gravity wins and the star contracts.
- 8Equal pulls keep it steady; unequal pulls move it.
Evolution and end stages of stars
Even a star that radiates for ages runs out of fuel, and its fate depends on its mass.
The change in a star's properties with time is its evolution (उत्क्रांती). The more massive the star, the faster it evolves. By initial mass a star ends as a white dwarf (श्वेत बटू), a neutron star (न्यूट्रॉन तारा) or a black hole (कृष्णविवर).
Rule 1: Mass and the end stage — Below 8 MSun: white dwarf. 8 to 25 MSun: neutron star. Above 25 MSun: black hole.
- Easy : The Sun will end as a white dwarf.
- Medium : A 15 MSun star becomes a neutron star.
- Exam-type : A 40 MSun star keeps contracting after its supernova and becomes a black hole.
Evolution and end stages of stars
In the app you can move this figure with your finger.
Open in the appRule 2: The order of stages — A low-mass star becomes a red giant, throws off its outer layers and is left as a white dwarf. A high-mass star becomes a supergiant, explodes as a supernova, and leaves a neutron star or a black hole.
- Easy : A white dwarf is about the size of the Earth.
- Medium : A neutron star is about 10 km across.
- Exam-type : Even light cannot escape a black hole, so it cannot be seen.
Rule 3: Red giants and dense remnants — A red giant's radius grows 100 to 200 times, and it looks reddish because it is cooler. White dwarfs and neutron stars are extremely dense.
- Easy : As a red giant the Sun will swallow Mercury and Venus.
- Medium : A spoonful of white dwarf matter weighs a few tonnes.
- Exam-type : Electron pressure does not depend on temperature, so a white dwarf stays stable for ever; in a neutron star, neutrons do this.
The end stage depends on the star's initial mass.
The Sun will not become a black hole; its mass is below 8 MSun, so it becomes a white dwarf.
Memory clue : "8 and 25: W – N – B" — below 8 white dwarf, 8–25 neutron, above 25 black hole.
Spot the difference
| Initial mass | End stage |
|---|---|
| Below 8 MSun | White dwarf |
| 8 to 25 MSun | Neutron star |
| Above 25 MSun | Black hole |
Practice questions
- 1How will the Sun end?
- 2Why is a black hole not visible?
- 3What is evolution of a star?
- 4A 10 MSun star ends as what?
- 5Why does a red giant look red?
- 6Which burns out sooner: more or less massive?
- 7Why does a white dwarf stay stable for ever?
- 8When does a black hole form after a supernova?
Show answers (try them yourself first)
- 1As a white dwarf.
- 2Even light cannot escape it.
- 3Change in a star's properties with time.
- 4A neutron star.
- 5Because it is cooler.
- 6The more massive.
- 7Electron pressure does not depend on temperature.
- 8When mass is above 25 MSun and no pressure can stop gravity.