The universe Cepheids Standard candles Commentary

Henrietta Swan Leavitt, Harlow Shapley, Edwin Hubble, Walter Baade astronomy

The universe

• Stars come in all sizes,
• so the brightness of a star alone
• tells us nothing about how far away it is.
• Henrietta Swan Leavitt studied photographic plates
• of 1777 stars in the Magellanic Clouds
• whose brightness varied regularly,
• called cepheid variable stars.
• Assuming they were all about the same distance from us,
• Leavitt discovered that brigher ones had longer periods;
• in fact, she found that the logarithms of their periods
• is directly related to their brightnesses.
• Leavitt’s discovery let Harlow Shapley use the periods
• and brightnesses of cepheids to measure the size of our galaxy, the Milky Way,
• and to locate our solar system in the galaxy.
• The big debate was whether spiral nebulae were inside the Milky Way
• or were similar in size as the Milky Way but very far away.
• This debate was settled when Edwin Hubble discovered cepheids
• in the Andromeda Galaxy, since their periods and brightnesses
• let him determine how far away they were.
• Walter Baade was the first to recognize different types of cepheids,
• some younger and brighter, and some older and less bright,
• which helped him double Hubble’s estimate for the size of the universe.

Cepheids

• The cepheid stars are named after the star Delta Cephei
• in the constellation Cepheus
• identified by John Goodricke in 1784.
• Delta Cephei is in the Milky Way
• about thirteen hundred light-years from us,
• the Milky Way being a hundred thousand light-years in diameter.
• The Milky Way has under four hundred billion stars.
• The observable universe has over ten sextillion stars.
• Obviously we don’t have names for them all.

Standard candles

• The mind is a candle
• that burns over many years, determining
• the size, the distance, and the importance
• of everything in its narrow scope.
• But it’s often wrong.
• Unacceptable events disturb it;
• contrary evidence confuses it;
• self-calibration is difficult.
• When a window opens
• it flickers in the wind.
• Useless in the sunlight;
• better than nothing in the dark.