Man is so tiny in the Universe that celestial bodies might seem to be too remote to be within the reach of human beings. Yet ingenious astronomers throughout history still perseveringly hacked their way to measure the distances of these celestial bodies purely by their twinkling, obscure lights.
To measure the distance of heavenly objects close to us, we can use a method called "parallax", a simple visual phenomenon that was well understood by ancient Greek more than two thousand years ago and was then used to estimate the Earth's radius.
The Hubble Space Telescope captured the
light variation of o e of the Cepheids inside the spiral galaxy M100 which
is at a distance of 56 million light years away from Earth.
Photo Credit : NASA, HST, W. Freedman (CIW),
R. Kennicutt (U.Arizona), J. Mould (ANU).
In the 16th century, Danish astronomer Tycho Brahe applied the method to prove that comets were further away from the Earth than the Moon. Parallax is a reliable method to gauge the distance of celestial objects a few thousand light-years away. For objects outside that range, we have to find another way.
In 1784, John Gooddricke, a 19-year old British astronomer discovered that Delta Cephei was a variable star with a period of 5.4 days. Two years later, this dumb and deaf genius suddenly passed away. But his discovery offered the mankind a tool to measure stellar distance up to over 100 million light-years! Since Delta Cephei was one of the earliest members of a special kind of variables being discovered, all variables in this family are called Cepheid variables or simply Cepheids. Subsequently astronomers found more and more Cepheids in different corners of the Universe and discovered that there exists a simple relationship between their luminosities and light variation periods. By making use of this relationship, astronomers can turn Cepheids into celestial yardsticks of the Universe, enabling us to easily map the position of galaxies within 100 million light-years. However, the Universe is much bigger than that, the yardstick is still too short in comparison.
In the beginning of the 20th century, American astronomer Edwin Hubble discovered that nearly all galaxies are moving away from us with velocities proportional to their distances from the Earth, inferring that the Universe is expanding. Based on his discovery, astronomers can first find out the receding velocity of a celestial body by the redshift of its spectrum, then using the receding velocity-distance relationship, or the Hubble's law, to calculate the distance. In principle, this method can be applied to celestial object with any distance, no matter how far away. However, the Hubble's law is based on the Big Bang Theory which is still being developed, the accuracy of the distances obtained is questionable.
In recent year, astronomers found a new celestial yardstick, which allows us to measure more precisely the distances of celestial bodies billions of light-years away. When studying Type Ia supernovae, astronomers found that not only they have similar light curve, they also have the same peak luminosity. As supernova is the explosive death of a star in an event so violent that for a brief period that a single star shines as brightly as a whole galaxy of more than 100 billion ordinary stars, it can be easily seen even it is a few billion light-years away. Astronomers painstakingly scan galaxies in the Universe for Type Ia supernovae. If such supernovae are luckily found, the peak luminosities can be measured and hence the distances of the host galaxies from the Earth can be determined. It was through this method that a group of American astronomers worked out in 1998 the distance of some extremely remote galaxies. If we compare the distanced of these galaxies deduced from Type Ia supernovae with those from Hubble's law, it will provide extremely valuable information for astronomers to verify the Big Bang Theory.