SSC CGL Physics · Astronomy and Space

The Universe and Satellites

The steady state theory, pulsars and black holes, the James Webb Space Telescope, light pollution, what keeps a satellite in orbit, sun-synchronous orbits and space weather.

All Astronomy and Space notes

Why this matters

Seven questions in the bank: one from the 2023 shifts and six from the 2025 shifts. The 2025 ones are statement questions, so each fact here comes with the false version SSC set beside it.

Concept 1 of 2: Stars, telescopes and theories

A few objects and theories come up, each with one fact that decides the question.

Definition

The facts:

Pulsars are rapidly rotating neutron stars.
Nothing escapes a black hole's event horizon; its X-rays come from hot gas outside it.
The steady state theory (1948): Fred Hoyle, with Bondi and Gold. Hubble observed the universe expanding.
The James Webb Space Telescope sees in near- and mid-infrared, sits at the Sun-Earth L2 point and carries coronagraphs for exoplanets.
Light pollution hides faint deep-sky objects (galaxies, nebulae) most.

Item	Fact
Steady state theory	Fred HoyleQ
James Webb telescope	Infrared, at L2Q
Hurt most by light pollution	Deep-sky objectsQ

Practise this conceptself-check · 3 quick reps

The same idea in a real exam question:

SSC CGL · 2025 · Tier I, 16 Sep 2025, Shift 3 · Q48Moderate

Example 1 · Astronomy and Space · The Universe and Satellites

Consider these cosmic phenomena: 1. Black holes emit X-rays directly from their event horizon 2. Pulsars are rapidly rotating neutron stars. Which of the above is/are correct?

No X-rays from inside the horizon

A statement that black holes emit X-rays from their event horizon is false. The X-rays come from gas outside it.

Drill 3 more on stars, telescopes and theories

Concept 2 of 2: Satellites and space weather

A satellite stays up because it moves sideways fast enough that it keeps falling round the Earth. Different orbits suit different jobs.

Definition

Orbits:

Orbital velocity keeps a satellite in orbit without engines; gravity supplies the centripetal force. The satellite's mass does not matter.
A sun-synchronous orbit is near-polar and low; the satellite passes each place at the same local solar time.
A geostationary orbit is a different one: over the equator, about 36,000 km up, one day per orbit.

Space weather: solar flares and coronal mass ejections disrupt satellites and radio and damage power grids. They do not change ocean currents.

Item	Fact
Sun-synchronous orbit	Same local solar timeQ
Geostationary orbit	Equatorial, 36,000 km
Not caused by solar storms	Changes in ocean currentsQ

Practise this conceptself-check · 3 quick reps

The same idea in a real exam question:

SSC CGL · 2025 · Tier I, 12 Sep 2025, Shift 2 · Q50Easy

Example 2 · Astronomy and Space · The Universe and Satellites

What specific physical quantity ensures a satellite maintains its orbit without continuous propulsion?

Ocean currents are not space weather

Solar storms disturb radio, satellites and power grids. Ocean currents are driven by wind, rotation, temperature and salt.

Drill 2 more on satellites and space weather

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)

Stars, telescopes and theories3 rows

Item	Fact
Steady state theory	Fred HoyleQ
James Webb telescope	Infrared, at L2Q
Hurt most by light pollution	Deep-sky objectsQ

Satellites and space weather3 rows

Item	Fact
Sun-synchronous orbit	Same local solar timeQ
Geostationary orbit	Equatorial, 36,000 km
Not caused by solar storms	Changes in ocean currentsQ

Watch out for (2)

No X-rays from inside the horizon
→ Stars, telescopes and theories
Ocean currents are not space weather
→ Satellites and space weather

Test yourself on a real paper

Sit a past SSC CGL paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Sit an SSC CGL past paper Drill all 7 questions