Download Measuring the distance to Galaxies

Measuring the distance to
Galaxies
Henrietta Leavitt and the Cepheid
Variable Star
The Great Distances of Space
Many of the challenges in
understanding the universe have
involved measuring distances
 How can you measure the distance to
objects that are too far away to visit?

Inverse Square Law

brightness
100.0
90.0
80.0
70.0
brightness
The brightness of
the star changes
according to an
inverse square law
 When the star is
twice as far away, it
is 1/4 as bright
 3 times as far away,
it will be 1/9 as
bright
60.0
50.0
40.0
30.0
20.0
10.0
0.0
0
5
10
distance
15
QuickTime™ and a
BMP decompressor
are needed to see this picture.
Click animation to start
Distance
If you know the
amount of light that
a star emits, you
can use the inverse
square law to work
out its distance from
us by reading off the
graph
100.0
90.0
80.0
70.0
brightness

brightness versus distance
60.0
50.0
40.0
30.0
20.0
10.0
0.0
0
5
10
distance
15
Stars of equal brightness
The brightness of stars varies both
because of the luminosity and distance
 Luminosity is a way of measuring the
light emitted by a star, the more
luminous the star, the more light it emits

Are the three bright stars
equally distant - we cannot tell
from the photo unless we
know that they are equally
luminous
“Harvard Computer” Henrietta Leavitt
Some stars vary in luminosity a bit like a
lighthouse
 In 1912, Henrietta Leavitt discovered a
type of variable star for which the more
luminous the star, the longer the period of
variation in brightness
 These stars are called Cepheid Variables

Cepheid Variables

2.5
2
1.5
brightness
The varying
brightness of 2
Cepheid
variable stars
 The bottom
one (pink) is
twice as
luminous as
the top one
1
0.5
0
-0.5 0
5
10
15
-1
-1.5
time
20
25
30
These stars have equal maximum brightness
 Which of these two Cepheid Variables is
more luminous?
 Which of these stars is further away?
 Can you work out how many times further
away?
Answers
1.Which of these two
Cepheid Variables
is more luminous?
2. Which of these
stars is further
away?
3. Can you work out
how many times
further away?

The star on the right has a
longer period so must be
more luminous

The star on the right is more
luminous but appears only as
bright as the one on the left. It
must be further away
The period is doubled so it is
twice as luminous but its
brightness is halved. It is less
than twice as far away. (It will
be 1.4 times as far away).

If the two Cepheid Variables
were equally distant
QuickTime™ and a
BMP decompressor
are needed to see this picture.
A movie of variable stars in a
globular cluster
Finding the Distance to
Cepheids
The distance to nearby Cepheid
variables can be determined by parallax
(a method you will learn in this course)
 The inverse square law and the periodluminosity relationship of Henrietta
Leavitt enables the distance of all
observable Cepheid variables to be
determined

Measuring the distance to
galaxies
Cepheids are found in other galaxies
 Astronomers can therefore measure the
distance to other galaxies.

The Discovery of Galaxies


In the 1920s astronomers
questioned whether objects
like this were small nebulae
within our galaxy, or
enormous galaxies great
distances beyond our
galaxy.
Cepheid variables in the
objects showed that they
were huge distances from
our galaxy