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star common centre of mass 'planet

Figure 5.6 Planets orbiting other stars — the Doppler shift.

its composition and the wavelengths of that light will wobble around their known values, oscillating between lower (blue-shifted) values as it approaches us and higher (red-shifted) values as it recedes from us. The planet itself is not visible but we can 'see' its effect on the light emitted by the star. The effect is so small that very sensitive means of detection are needed.

Probably the best-known of all USPPs orbits a star called 51 Pegasi. It was first discovered in 1995, using the Doppler technique. The Jupiter-like planet races around the parent star in just four days, in an orbit about 16 th of the size of the orbit of

Mercury.

5.3.2 Other galaxies

The Milky Way is by no means the only cluster of stars in the universe. In 1924 the astronomer Edwin Hubble (1889-1953) proved that what were then known as extragalactic nebulae or clouds, such as the Andromeda Nebula, are in fact vast assemblies of stars, similar to our own galaxy. It takes light more than 100,000 years to cross from one side of such galaxies to the other.

The Large Magellanic Cloud, at about 170,000 light years from the Milky Way, is the nearest neighbour. The largest member of our 'local group' of four galaxies is M31 — the Andromeda galaxy — at about 2.2 million light years away.

M31 — Andromeda galaxy. Courtesy of NASA Marshall Space Flight Centre (NASA-MSFC).

HCG 87 group of galaxies. Courtesy of NASA/ESA.

M31 — Andromeda galaxy. Courtesy of NASA Marshall Space Flight Centre (NASA-MSFC).

HCG 87 group of galaxies. Courtesy of NASA/ESA.

(The many large bright stars in the picture are from our own galaxy, and are much nearer.)

Hubble was the first astronomer to realise that, on the scale of the universe, the Milky Way is insignificant, one of tens of billions of galaxies. We now know that there are more galaxies in the universe than there are stars in our galaxy. There are galaxies at distances of more than 20 billion light years from us.

In 1929, Hubble made the remarkable discovery that there was a systematic change in the wavelength of characteristic spectral lines of basic elements in the light emitted by other galaxies. (He found that the measured wavelengths were longer than expected; the lines were 'red-shifted'.) Attributing this to the Doppler effect, he concluded that practically all the galaxies are moving away from us. The more distant the galaxy, the faster it is moving. Galaxies 10 or 20 billion light years away are receding with speeds approaching the speed of light.

Hubble derived an equation connecting the recession speed v of a distant galaxy and its distance r from us. He combined his own distance measurements with red shifts measured by another American, Vesto Slipher, and established Hubble's law:

H is Hubble's constant, which he estimated to be 150 km per second per kpc; more recent estimates average out at about 75 km per second per kpc. (1 kpc = 3.09 x 1022 m.) There is no commonly accepted value for H.

The more distant the galaxy, the faster it moves away from us and the larger the red shift.

For stars which are closer to us, the red shift is smaller and is sometimes masked owing to the motion of the earth in its orbit about the sun.

A remarkable conclusion

The evidence that galaxies are moving away from us in such a systematic fashion points to an unavoidable conclusion:

The universe, i.e. the totality of space, mass and time observable by us, is expanding.

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