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6.2-Stars and the universe

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IGCSE Physics Notes / Stars and the Universe

 

Summary Fill in the Blanks

Click the blanks to reveal the key answers.

1The Sun as a Star

The Sun is a star of
_______medium size
.
It consists mostly of
_______hydrogen
and
_______helium
.
The Sun radiates most of its energy in the
_______infrared
,
_______visible
and
_______ultraviolet
regions of the electromagnetic spectrum.
Stars produce energy through
_______nuclear reactions
.
In stable stars these reactions involve the fusion of
_______hydrogen
into
_______helium
.

2Stars and Galaxies

Galaxies are made up of many
_______billions
of stars.
The Sun is a star in the galaxy called the
_______Milky Way
.
Other stars in the Milky Way are much
_______further away
from Earth than the Sun.
Astronomical distances are measured in
_______light-years
.
One light-year is the distance travelled by
_______light
in one
_______year
.
1 light-year equals approximately
_______9.5 × 10?1;? m
.

3Life Cycle of a Star

Stars form from interstellar clouds of
_______gas
and
_______dust
.
These clouds contain large amounts of
_______hydrogen
.
A collapsing cloud forming a star is called a
_______protostar
.
A stable star forms when
_______gravitational attraction
is balanced by outward pressure from
_______high temperature
in the core.
Eventually stars run out of
_______hydrogen
fuel.
Most stars expand to form
_______red giants
.
More massive stars form
_______red supergiants
.
A red giant may form a
_______planetary nebula
with a
_______white dwarf
at its centre.
A red supergiant may explode as a
_______supernova
.
This can leave behind a
_______neutron star
or a
_______black hole
.
Supernova remnants can form new
_______stars
and
_______planets
.

4The Expanding Universe

The Milky Way is one of
_______billions
of galaxies in the Universe.
Its diameter is approximately
_______100000 light-years
.
Redshift is an increase in the
_______wavelength
of radiation from receding galaxies.
Light from distant galaxies appears
_______redshifted
.
Redshift provides evidence that the Universe is
_______expanding
.

5Big Bang Evidence

Microwave radiation observed throughout space is called
_______cosmic microwave background radiation
.
This radiation is abbreviated as
_______CMBR
.
The Hubble constant is defined as
_______velocity ÷ distance
.
The equation is
_______Ho = v / d
.
The current estimate for H? is approximately
_______2.2 × 10??1;? s??1;
.
The equation
_______d / v = 1 / H?
gives an estimate for the
_______age of the Universe
.

Exam Summary

Stars produce energy by nuclear fusion.

Galaxies contain billions of stars.

Distances in space are measured in light-years.

Stars have a life cycle ending in white dwarfs, neutron stars or black holes.

Redshift and CMBR provide evidence for the Big Bang and an expanding Universe.

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Syllabus Objectives

6.2.1 The Sun as a star

CORE OBJECTIVES:

  • Know that the Sun is a star of medium size, consisting mostly of hydrogen and helium, and that it radiates most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum

EXTENDED/SUPPLEMENT  OBJECTIVES:

  •  Know that stars are powered by nuclear reactions that release energy and that in stable stars the nuclear reactions involve the fusion of hydrogen into helium

6.2.2 Stars

CORE OBJECTIVES:

  • State that:
  • (a) galaxies are each made up of many billions of stars
  • (b) the Sun is a star in the galaxy known as the Milky Way
  • (c) other stars that make up the Milky Way are much further away from the Earth than the Sun is from the Earth
  • (d) astronomical distances can be measured in light-years, where one light-year is the distance travelled in (the vacuum of) space by light in one year

EXTENDED/SUPPLEMENT  OBJECTIVES:

  •  Know that one light-year is equal to 9.5 × 1015m
  • Describe the life cycle of a star:
  • (a) a star is formed from interstellar clouds of gas and dust that contain hydrogen
  • (b) a protostar is an interstellar cloud collapsing and increasing in temperature as a result of its internal gravitational attraction
  • (c) a protostar becomes a stable star when the inward force of gravitational attraction is balanced by an outward force due to the high temperature in the centre of the star
  • (d) all stars eventually run out of hydrogen as fuel for the nuclear reaction
  • (e) most stars expand to form red giants and more massive stars expand to form red supergiants when most of the hydrogen in the centre of the star has been converted to helium
  • (f) a red giant from a less massive star forms a planetary nebula with a white dwarf star at its centre
  • (g) a red supergiant explodes as a supernova, forming a nebula containing hydrogen and new heavier elements, leaving behind a neutron star or a black hole at its centre
  • (h) the nebula from a supernova may form new stars with orbiting planets

6.2.3 The Universe

CORE OBJECTIVES:

  • Know that the Milky Way is one of many billions of galaxies making up the Universe and that the diameter of the Milky Way is approximately 100000 light-years 
  • Describe redshift as an increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies
  •  Know that the light emitted from distant galaxies appears redshifted in comparison with light emitted on the Earth
  • Know that redshift in the light from distant galaxies is evidence that the Universe is expanding and supports the Big Bang Theory

EXTENDED/SUPPLEMENT  OBJECTIVES:

  •  Know that microwave radiation of a specific frequency is observed at all points in space around us and is known as cosmic microwave background radiation (CMBR)
  •  Explain that the CMBR was produced shortly after the Universe was formed and that this radiation has been expanded into the microwave region of the electromagnetic spectrum as the Universe expanded
  •  Know that the speed v at which a galaxy is moving away from the Earth can be found from the change in wavelength of the galaxy’s starlight due to redshift
  •  Know that the distance of a far galaxy d can be determined using the brightness of a supernova in that galaxy
  •  Define the Hubble constant H0 as the ratio of the speed at which the galaxy is moving away from the Earth to its distance from the Earth; recall and use the equation 
  •  Know that the current estimate for H0 is 2.2 × 10–18 per second
  • Know that the equation  represents an estimate for the age of the Universe and that this is evidence for the idea that all the matt

 

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