October is an ideal time to discover the star Delta, an astronomical treasure found in Cepheus, a constellation that looks fairly ordinary at first glance.
Cepheus lies in the northeast in the evening at this time of year. To find it, first look high in the sky for the famous “W” of the constellation Cassiopeia. If you extend the line connecting the two stars in the right branch of the “W” three times, you’ll reach the centre of a group of stars forming a little upside-down house with a pointed roof. The star Delta Cephei is located at the foot of this house, on the Cassiopeia side. With an average magnitude of +4, it’s easily visible with the naked eye in the countryside. In the city, use a pair of binoculars, especially if there’s a lot of light pollution.
In 1784, an English amateur astronomer named John Goodricke (1764-1786) discovered that Delta Cephei is a variable star, meaning that its intrinsic luminosity varies over time. Astronomers later realized that several other stars also have variations in luminosity similar to Delta Cephei, and so this star became the prototype of an entire class of variable stars called Cepheids.
Cepheids are very bright giant or supergiant stars whose size and mass are several times greater than the Sun’s. The variation in luminosity of Cepheids is due to the regular pulsation of the star’s outer layers. Their pulsation period lasts between one and 135 days.
As for Delta Cephei, it’s a supergiant four and a half times more massive and 2,000 times more luminous than the Sun, with a diameter 45 times larger than our star’s. It’s just under 900 light-years from Earth, and its luminosity varies over a period of five days, eight hours and 47 minutes (5.366 days).
Back in 1912, Cepheids earned their stripes thanks to the work of American astronomer Henrietta Swan Leavitt (1868-1921). Leavitt, an expert in variable stars, determined that the longer a Cepheid’s pulsation period is, the greater its intrinsic luminosity will be.
Therefore, by measuring a Cepheid’s period, we can deduce its absolute luminosity; by comparing this with its apparent brightness, we can calculate the star’s distance. If the Cepheid is located in another galaxy, we can then figure out the distance of the galaxy. A body whose absolute luminosity enables us to determine the distance of other celestial bodies is called a “standard candle.”
Determining distances with standard candles
Today, we can make out Cepheids up to a hundred million light-years away. They can be used to determine the distance of galaxies closest to our own.
In 1917, astronomer Harlow Shapley (1885-1972) demonstrated, through a study of Cepheids, that the Sun lies not at the heart of the Milky Way but farther off, about 26,000 light-years from our galaxy’s centre.
In 1923, astronomer Edwin Hubble (1889-1953) discovered Cepheid stars in what was then called the “Andromeda nebula,” proving that it was an actual galaxy. Therefore, the Universe wasn’t limited to the Milky Way alone and was vastly larger than we’d ever imagined.
Planets visible in October
Jupiter makes a final appearance, showing up above the southwestern horizon about a half hour after sunset. The giant planet vanishes in the Sun’s glow in November before reappearing in December in the morning sky. On October 11 at twilight, the crescent Moon is about 3 degrees to the upper right of Jupiter. The duo sets just before 8 p.m.
On the evening of October 14, the Moon lies near Saturn. Starting at 7 p.m., you can find the planet less than 2 degrees to the left of our natural satellite in the southwest. The two bodies are observable till they set around 10 p.m.
Finally, Mars, which stands out thanks to its reddish-orange glow, lights up October evenings in the southeast and sets around 1 a.m. On October 17, a quarter Moon shines 6 degrees to the right of the red planet. The next evening, the Moon appears to the left of Mars.