Exam practice

Stars are born in nebulae. Within a nebula, gravity pulls gas and dust together into protostars, which become stars when fusion begins. So, like a nursery where babies are cared for, a nebula is where new stars form; the Orion Nebula has thousands of young stars being born in it.

A star’s color depends on its surface temperature: the hottest stars glow blue, then white, yellow and orange, and the coolest red. Stars have different temperatures because they have different masses (massive stars are hotter) and are at different stages of their life (an old star that swells into a red giant cools at the surface and turns red).

A black hole has so much mass squeezed into so small a space that its gravity is strong enough to stop even light from escaping. We see things only by the light that comes from them, so a black hole gives out no light and can’t be seen. It is found only by its effects on stars and gas near it.

Like a star, Jupiter is made mainly of hydrogen and helium, the same as the Sun. But it doesn’t have enough mass (it would need about 80 times as much) for its core to get hot enough and squeezed enough for hydrogen fusion to start. So it never became a star. (Strictly, it is a true planet, formed in a different way from a star, but its make-up is why people call it a failed star.)

A nebula is a giant cloud of gas and dust in space, made mostly of hydrogen and helium, in which stars are born. Some of its material came from earlier stars that exploded. Example: the Orion Nebula.

A black hole is what is left when the core of a very massive star collapses after a supernova: a huge mass squeezed into almost no volume, whose gravity is so strong that nothing, not even light, can escape from it. It is invisible, and is found by the stars moving around it and the X-rays given out by hot gas falling into it.

(Draw the flowchart in Figure 12.3.)

1. Nebula: a cloud of hydrogen, helium and dust.
2. Protostar: gravity pulls part of the nebula into a ball, which heats up as it shrinks.
3. Star: at about 10⁷ K, hydrogen fusion starts; the star shines steadily for millions to billions of years.
4. When the core’s hydrogen runs out, helium fuses into carbon, and the star swells into a red giant (or a red supergiant if it is very massive).
5. A star like the Sun throws off its outer layers (as a planetary nebula, called a nova in some books) and its core becomes a white dwarf, which slowly cools into a black dwarf.
6. A massive star explodes as a supernova. Its core becomes a neutron star (seen as a pulsar) or, if heavy enough, a black hole.
7. The gas and dust thrown out form new nebulae, from which new stars are born.

In about 5 billion years, the hydrogen in the Sun’s core will run out. The core will shrink and heat up until helium fuses into carbon. The extra heat will make the Sun’s outer layers swell to perhaps 200 times their present size: the Sun will become a red giant, swallowing Mercury and Venus and possibly the Earth. Its outer layers, weakly held, will then drift off into space as a glowing shell of gas. The core left behind will be a white dwarf, about the size of the Earth, hot but dim, with no fusion. Over an extremely long time it will cool and fade into a cold, dark black dwarf. The Sun is not massive enough to explode as a supernova or become a black hole.

1. The universe began with the Big Bang about 13.8 billion years ago, and it is still expanding.
2. It is made of matter and energy; the matter we can see is only about 5 % of it (the rest is dark matter and dark energy).
3. Nothing in it is at rest: the Moon goes round the Earth, the Earth round the Sun, the Sun round the center of the Milky Way.
4. There are hundreds of billions of galaxies; the nearest large one, Andromeda, is 2.5 million light-years away.
5. The Solar System formed about 4.6 billion years ago, in a spiral arm of the Milky Way; the oldest pieces of the Earth, zircon crystals, are about 4.4 billion years old.
6. Planets similar to the Earth have been found around other stars, such as TOI 700 d.
7. Stars are born, live and die, and their matter is reused to make new stars and planets.

The distances between celestial bodies are enormous. In meters or kilometers they become huge numbers that are hard to write, read and compare (the nearest star is about 40,000,000,000,000 km away). Bigger units make them simple: the Sun is 1 AU away, Mars 1.5 AU; Proxima Centauri is 4.2 light-years away. The AU (1.5 × 10¹¹ m) suits distances within the Solar System, and the light-year (9.46 × 10¹⁵ m) the distances between stars and galaxies.

Cosmology and astronomy are still young sciences in Nepal, but growing:

• NAST (Nepal Academy of Science and Technology) promotes research in science and technology, is working to set up a National Space Research Centre, and led Nepal’s first satellite, NepaliSat-1 (2019).
• Universities such as Tribhuvan University teach astronomy and astrophysics within physics, and some students go on to research abroad.
• Amateur astronomy groups hold star-gazing events and public talks, and Nepal’s dark skies in the mountains are excellent for observing.
• Nepal named a star (Sagarmatha) and its planet (Laligurans) in the IAU’s 2019 naming project.
• Much remains to be done: Nepal has no large research telescope yet, and more funding, equipment and trained people are needed.

NASA, the space agency of the United States, set up in 1958, has been a leader in space exploration:

• it landed the first people on the Moon (Apollo 11, 20 July 1969);
• with other countries’ agencies, it built and runs the International Space Station, a laboratory in orbit;
• its space probes have visited every planet, studied moons, asteroids and comets, and the Parker Solar Probe has flown through the Sun’s corona;
• its rovers explore Mars for signs of past life (Perseverance, 2021);
• its space telescopes, Hubble (1990) and James Webb (2021), have shown us distant galaxies, the birth of stars and planets around other stars;
• it shares its pictures and data freely with scientists and the public all over the world.