September Sky — Nebulae, The Dying Stars
In the September sky, we’ll be exploring the Pisces and Aquarius constellations to observe dying stars known as Nebulae. When stars are born, they consist mainly of hydrogen atoms. Over time, the core of a star changes as hydrogen fuses into helium, increasing the core pressure. This process continues with heavier elements forming as the star ages. Many elements that make up our bodies were created in distant stars and later entered our solar system, including water.
As hydrogen fuses to form helium in a star’s core, the pressure from energy production pushes the core outward, while gravity tries to pull it inward, creating an equilibrium. This balance lasts for billions of years. During this process, stars emit light, and by studying their spectra, we can learn about their composition and life cycle.
When stars with less than 9 times the mass of the Sun exhaust their hydrogen fuel, fusion fades, and gravity compresses the core. This increase in temperature and pressure reignites the fusion process, turning helium into carbon, causing the star to swell up. For stars like our Sun, this stage would eventually lead to its outer layers engulfing Mercury and Venus, becoming a red giant, and eventually, a white dwarf. Betelgeuse in Orion is an example of a red supergiant visible clearly from November in the Northern Hemisphere.
Variable stars are fascinating because they vary in brightness over time. Some variable stars change their light emission, while others are blocked by objects before them. Even our Sun’s brightness varies slightly about 0.1% due to changes in its distance from us.
In the Northern Hemisphere, if we look towards north from Pisces we can identify Andromeda, our nearest galaxy. Southwest of it, we can find Algol in the Perseus constellation. Algol is a variable star whose brightness changes every 3 days due to its companion star periodically blocking its light.
Cetus constellation lies in the south of Pisces. Mira, a binary star in the Cetus constellation in southern hemisphere, was the first variable star discovered in the 17th century. Mira A is a red dwarf nearing the end of its life, and Mira B is a white dwarf with higher gravity. Some red dwarfs may transfer material to white dwarfs, and if the mass exceeds a critical point, it can lead to a type 1 supernova explosions. Such explosions are common in the sky.
Moving north in September, we can find Gamma Piscium, the brightest star in Pisces. Towards the north between the two fish, we see a large square formation called Pegasus, with Algenib, Markab, Scheat, and Alpheratz at its corners. Alpheratz is actually a member of Andromeda, not Pegasus. The red star Enif marks the end of Pegasus.
Heading north from Andromeda and Perseus, we come across Cassiopeia, with Caph helping us locate Caroline’s Rose, an open-star cluster (NGC 7789).
In the vicinity of Cassiopeia, we find Cepheus, home to the famous Delta Cephei (Alfirk), which serves as a benchmark for Cepheid Variable Stars. Alderamin is the brightest star in Cepheus, and nearby, we have mu-Cephei or Herschel’s Garnet Star, which is a deep red supergiant, one of the largest stars in our galaxy.
For most average-sized stars, the process of fusion is repeated over time, leading to the formation of heavier elements. As the fusion reactions progress, the star’s core pulsates until it reaches an intense stage, eventually culminating in a spectacular explosion known as a planetary nebula. Planetary nebulas appear as circular and planet-like structures in the sky.
Two well-known examples of planetary nebulae are the Ring Nebula in the constellation Lyra and the Helix Nebula in Aquarius. Just to the north of Pegasus lies the constellation Cygnus. Moving further north(east), we find the Lyra constellation, which contains the bright star Vega. The Ring Nebula, an excellent example of a planetary nebula, can be observed with telescopes.
For stars above 9 times the mass of the Sun, their core fusion produces elements beyond carbon, leading to the formation of neon, oxygen, silicon, and iron in a short period. When iron is produced, energy generation stops, and electron degeneracy pressure tries to prevent core collapse. The core mass increases to 1.4 time the mass of the sun.
In stars with masses over 20 times that of the Sun, neutron degeneracy pressure comes into play, resulting in the core collapsing to form a neutron star. In some cases, the outer layers of the star explode in a type 2 supernova.
The Cygnus constellation contains Deneb, its brightest star, and between Zeta Cygnus and Gienah lies the Veil Nebula, a remnant of a type 2 (visible to naked eyes under dark sky) supernova explosion that occurred 7000 years ago.
Moving west from the Aquarius constellation, we find Fomalhaut in Piscis Austrinus, the brightest star about 25 light-years away from us. Near Fomalhaut, we can observe the Helix Nebula (Caldwell 63), the closest nebula at a distance of 650 light-years.
Other constellations in the area include Sculptor to the west, Phoenix to the south of Sculptor, and Grus to the east. Tucana celebrates the discovery of the Toucan bird in the 17th century. The Small Magellanic Cloud (SMC) is visible southwest of Tucana. Though it appears to be a detached piece of the Milky Way, it is actually 200,000 light-years beyond our galaxy. The Magellanic Clouds are two irregular dwarf galaxies orbiting the Milky Way as part of the Local Group, and they can be seen only from the Southern Hemisphere (not from the most northern latitudes.). SMC is our satellite galaxy.
Stars with masses between 20 and 40 times that of the Sun collapse even further, becoming incredibly dense objects called black holes. One strong candidate, Cygnus X-1, was found near the Eta Cygni in the Cygnus constellation.
