CDS Physics · Gravitation
Stars and Space
The stars nearest to us, what a constellation really is, the Lagrange points of the Sun and the earth, and India's Giant Metrewave Radio Telescope.
Why this matters
Four CDS questions, two in the 2019 (I) paper and two in the 2024 papers. They are pure recall: one named fact each, with distractors that sound just as likely.
Concept 1 of 2: Nearby stars and constellations
After the Sun, the nearest star is so far away that its light takes more than four years to reach us. The brightest star in the night sky is not the nearest one: Sirius looks brighter partly because it gives out far more light. A constellation is a picture our eyes join up on the sky, but its stars can be at very different distances from us; they only look close together because they lie in nearly the same direction.
Definition
Facts to know:
- A light year is a distance: how far light travels in one year, about m.
- The Sun is the nearest star to the earth; its light takes about 8 minutes 20 seconds to reach us.
- After the Sun, the nearest star is Proxima Centauri, about 4.24 light years away, part of the Alpha Centauri system.
- Sirius (about 8.6 light years) is the brightest star of the night sky, but not the nearest.
- A constellation is a pattern of stars as seen from the earth; its stars need not be equidistant from us. The International Astronomical Union recognises 88 constellations.
| Star or term | Fact |
|---|---|
| Sun | The nearest star; light takes about 8 min 20 s to reach us |
| Proxima Centauri | Nearest star after the Sun, about 4.24 light yearsQ CDS 2019 (I): of Sirius, Arcturus, Spica and Proxima Centauri, the nearest is Proxima Centauri. |
| Sirius | Brightest star of the night sky, about 8.6 light years |
| Arcturus and Spica | Bright stars much farther away (tens to hundreds of light years) |
| Constellation | A pattern of stars on the sky; the stars may be at very different distancesQ CDS 2019 (I): a constellation is a pattern of stars that may not be equidistant from the earth. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2019 · CDS (I) 2019 — General Knowledge · Q57Easy
Example 1 · Gravitation · Stars and Space
Which one among the following stars is nearest to the earth?
Brightest is not nearest
Sirius is the brightest star at night, but Proxima Centauri is the nearest after the Sun. Brightness depends on how much light a star gives out as well as on its distance.
A constellation is not a group in space
Its stars are joined up only as seen from the earth. Options saying the stars are equidistant from us, or that the pattern includes planets, are wrong.
The Sun is a star too
If a question asks for the star nearest to the earth with the Sun among the options, the answer is the Sun. Proxima Centauri is the nearest other star.
Concept 2 of 2: Lagrange points and the GMRT
Around two big bodies like the Sun and the earth there are five places where a small spacecraft can stay in step with them, because the two pulls and its circular motion balance. India parks its solar observatory at one of them. On the ground, near Pune, India listens to the sky at radio wavelengths with an array of thirty large dish antennas that work together as one giant telescope.
Definition
The two facts CDS asks:
- Lagrange points L1 to L5 of a two-body system (such as the Sun and the earth): points where a small body keeps its position relative to the two.
- L1, L2, L3 lie on the line through the two bodies. L1 is between them (Aditya-L1 orbits the Sun-earth L1, about 1.5 million km from the earth); L2 is beyond the earth (the James Webb Space Telescope).
- L4 and L5 lie 60° ahead of and behind the earth in its orbit. Each forms an equilateral triangle with the Sun and the earth.
- The Giant Metrewave Radio Telescope (GMRT), at Khodad near Narayangaon, about 80 km north of Pune, is an array of 30 fully steerable parabolic dishes, each 45 m across, working at metre wavelengths. It is run by the National Centre for Radio Astrophysics (TIFR).
| Place or instrument | Fact |
|---|---|
| Sun-earth L1 | On the Sun-earth line, between them; home of Aditya-L1 |
| Sun-earth L2 | On the line, beyond the earth; home of the James Webb Space Telescope |
| Sun-earth L4 (and L5) | Sun, earth and the point form an equilateral triangleQ CDS 2024 (II): the Sun, the earth and L4 form an equilateral triangle. |
| GMRT | 30 parabolic dishes, each 45 m across, near PuneQ CDS 2024 (I): the GMRT consists of multiple parabolic antennas. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2024 · CDS (II) 2024 — General Knowledge · Q30Hard
Example 2 · Gravitation · Stars and Space
In Sun-Earth system, the Sun, the Earth and the Lagrange point L4 form :
L4 and L5 are off the line
Only L1, L2 and L3 lie on the straight line through the two bodies. L4 and L5 make equilateral triangles with them, not isosceles or scalene ones.
The GMRT is many dishes, not one
The GMRT is an array of 30 parabolic dishes whose signals are combined. Options of a single dish, or of circular antennas, are wrong.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Nearby stars and constellations5 rows
| Star or term | Fact |
|---|---|
| Sun | The nearest star; light takes about 8 min 20 s to reach us |
| Proxima Centauri | Nearest star after the Sun, about 4.24 light yearsQ CDS 2019 (I): of Sirius, Arcturus, Spica and Proxima Centauri, the nearest is Proxima Centauri. |
| Sirius | Brightest star of the night sky, about 8.6 light years |
| Arcturus and Spica | Bright stars much farther away (tens to hundreds of light years) |
| Constellation | A pattern of stars on the sky; the stars may be at very different distancesQ CDS 2019 (I): a constellation is a pattern of stars that may not be equidistant from the earth. |
Lagrange points and the GMRT4 rows
| Place or instrument | Fact |
|---|---|
| Sun-earth L1 | On the Sun-earth line, between them; home of Aditya-L1 |
| Sun-earth L2 | On the line, beyond the earth; home of the James Webb Space Telescope |
| Sun-earth L4 (and L5) | Sun, earth and the point form an equilateral triangleQ CDS 2024 (II): the Sun, the earth and L4 form an equilateral triangle. |
| GMRT | 30 parabolic dishes, each 45 m across, near PuneQ CDS 2024 (I): the GMRT consists of multiple parabolic antennas. |
Watch out for (5)
- Brightest is not nearest→ Nearby stars and constellations
- A constellation is not a group in space→ Nearby stars and constellations
- The Sun is a star too→ Nearby stars and constellations
- L4 and L5 are off the line→ Lagrange points and the GMRT
- The GMRT is many dishes, not one→ Lagrange points and the GMRT
Test yourself on a real paper
Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.