Science notes, chapter 11

Stars and galaxies in the universe

Textbook Science Form 2 (DLP), pages 238–249

What this chapter covers

Define a galaxy and name the three types of galaxy with examples (11.1.1)
State where the solar system is located in the Milky Way (11.1.1)
Describe the birth of a star from a nebula and the death of stars of different sizes (11.1.1)
Compare the relative sizes of the Earth, solar system, Milky Way and universe (11.1.1)
Classify stars by colour, temperature, size, brightness and distance from Earth (11.1.2)
Relate the colour of a star to its surface temperature, including the Sun (11.1.2)

11.1Stars and galaxies in the universe

The universe is everything that exists in space: galaxies, stars, planets and all the space between them. Astronomy is the study of objects in space.

Technology helps us to see space more clearly. The Hubble space telescope, launched on 24 April 1990, is so powerful that it could see a coin from 725 km away. Dato' Dr. Sheikh Muszaphar Shukor was the first Malaysian in outer space (10 October 2007).

Galaxies (11.1.1)

A galaxy is a huge set of bodies made up of millions of stars together with gas and dust particles.

There are millions of galaxies in the universe. They have different forms.

Photographs of a spiral galaxy, an elliptical galaxy and an irregular galaxy with their examples — Types of galaxies.

Textbook figure: Science Form 2 (DLP), Photograph 11.2, p. 241.

Type	Examples
Spiral	Andromeda, the Milky Way
Elliptical	Ursa Major, Messier 87
Irregular	Small Magellanic Cloud, Large Magellanic Cloud

The Milky Way

The Milky Way is a medium-large spiral galaxy.
It has about 200 billion stars. The Sun is one of them.
Our solar system is at the edge of one of the spiral arms of the Milky Way.

The Milky Way is a galaxy, not a star and not the solar system. The solar system (the Sun and its eight planets) is only a tiny part inside the Milky Way.

Relative sizes in the universe

From smallest to largest: Earth, solar system, Milky Way, a group of galaxies, a cluster of galaxies, the universe. Compared with the universe the Earth is smaller than a speck of dust.

The Earth inside the solar system, inside the Milky Way, inside a group of galaxies, inside the universe — Relative sizes of the Earth, the solar system, the Milky Way, galaxies and the universe.

Textbook figure: Science Form 2 (DLP), Figure 11.2, p. 244.

Our solar system is found in which galaxy, and in which part of it?

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In the Milky Way galaxy, at the edge of one of its spiral arms.

Life cycle of a star (11.1.1)

A nebula is a large cloud of dust particles and gases such as hydrogen and helium. Stars are born from nebulae.

Birth of a star

Gravitational force pulls the gas and dust in a nebula together into a globe.
The globe shrinks and is compressed into a very dense core, because gravity gets stronger as it shrinks.
The temperature and pressure in the core become very high, so a nuclear reaction starts: hydrogen turns into helium and a huge amount of heat and light is released.
The core shines. The new star is called a protostar. It becomes an average star like the Sun or a massive star.

Death of a star

Heat from the star heats its outer layer, and hydrogen there begins to burn. The star expands and looks red: it is a red giant. What happens next depends on the size of the original star.

Original star	Stages	End product
Medium-sized (e.g. the Sun)	Nebula, star, red giant	White dwarf
Large	Nebula, star, red giant, supergiant, supernova	Neutron star
Super-large	Nebula, star, red giant, supergiant, supernova	Black hole

Life cycle of a star: a nebula forms a medium-sized star, then red giant, then white dwarf; a large star goes through red giant, supergiant and supernova to a neutron star; a super-large star goes through red giant, supergiant and supernova to a black hole — The life cycle of a star (nebular hypothesis).

Textbook figure: Science Form 2 (DLP), Figure 11.1, p. 242.

A supernova is a very big explosion of a red giant that is big enough. It is extremely bright and can be seen in daylight.
A black hole is so named because light cannot escape from it, and any matter that enters it cannot escape either.

The Sun is a medium-sized star, so it will end as a white dwarf. It will not explode as a supernova and will not become a black hole.

Remember the sizes with "Medium, Large, Super-large" = "White dwarf, Neutron star, Black hole" in the same order.

What happens to a red giant that is not massive?

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It forms a white dwarf. Only a red giant that is big enough contracts quickly and explodes as a supernova.

Characteristics of stars (11.1.2)

Stars can be classified by their colour, temperature, size, brightness and distance from Earth.

Colour and surface temperature

The colour of a star shows its surface temperature. Blue stars are the hottest and red stars are the coolest.

Colour	Surface temperature (K)
Red	below 3 500
Orange	3 500 – 5 000
Yellow	5 000 – 6 000
Yellowish-white	6 000 – 7 500
White	7 500 – 11 000
Bluish-white	11 000 – 25 000
Blue	above 25 000

Worked example

A star has a surface temperature of 9 000 K. What colour is it?
9 000 K lies between 7 500 K and 11 000 K, so the star is white.

The Sun is a yellow star. Its surface temperature, about 5 500 K, lies in the yellow range.

Do not link "red" with "hot" as in everyday life. For stars, red is the coolest and blue is the hottest.

Size and brightness

By size, stars are supergiant (the really big ones), giant and dwarf (the really small ones).
The brightness of a star depends on its size, its distance from Earth and its surface temperature.
Sirius and Rigel are the brightest stars in the sky.

A star that looks bright is not always a star that is really bright. A small star that is very near Earth can look brighter than a huge star that is very far away. That is why the Sun, the nearest star, looks so big and bright.

Characteristic	What it tells us
Colour	Surface temperature of the star
Size	Dwarf, giant or supergiant
Distance from Earth	Nearer stars look brighter
Brightness	Depends on size, distance and temperature

Star P is blue and star Q is red. Which is hotter? Can you tell which is bigger from the colour alone?

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P (blue) is hotter than Q (red). No. Colour tells us the surface temperature only, not the size.