The oldest stars are typically Population II stars, which are metal-poor and formed early in the universe, like the famous "Methuselah Star" (HD 140283), a metal-poor subgiant (spectral type G0IV-V) known for its extreme age estimates. While red dwarfs are theoretically the longest-lived, we've observed Population II stars with reliable ages, characterized by low "metallicity" (few elements heavier than hydrogen/helium).
HD 140283 (also known as the Methuselah star) is a metal-poor subgiant star about 200 light years away from the Earth in the constellation Libra, near the boundary with Ophiuchus in the Milky Way galaxy. Its apparent magnitude is 7.205, so it can be seen with binoculars. It is one of the oldest stars known.
The 7 main types of stars, classified by temperature and spectral class (O, B, A, F, G, K, M), range from hottest (O, blue) to coolest (M, red); these classes are further divided by luminosity, with our Sun being a G-type main-sequence star, while other categories include Red Giants, Supergiants, White Dwarfs, Neutron Stars, and Brown Dwarfs, showing diverse sizes, temperatures, and life stages.
Did you know? The oldest known star in the universe is HD 140283, also known as Methuselah. It is estimated to be approximately 14.5 billion years old, which overlaps with the age of the universe.
Protostar: The early stage of a star's formation, where nuclear fusion begins. Brown Dwarf: A failed star that never ignites nuclear fusion fully. Main Sequence Star: A stable phase where the star fuses hydrogen into helium. Red Giant: An expanded, cooling star running out of hydrogen fuel.
A Star's Seven Main Stages or Star Formation Stages are as follows:
The least luminous O-type stars can remain on the main sequence for around 10 million years, but cool slowly during that time and become early B-type stars. No massive star remains with spectral class O for more than about 5–6 million years.
This cycle, unique to T CrB, occurs roughly every 80 years. Recent observations show T CrB is mimicking the behavior seen before its last explosion in 1946, suggesting the nova could erupt as early as September 2024. Once it does, the star will shine brightly for about a week before fading.
For all we know, galaxies like the Andromeda Galaxy could be teeming with habitable worlds, some of which may even host intelligent life. But until definitively detected, for now the question as to whether life exists in the Andromeda Galaxy remains unanswered.
More massive stars must burn fuel at a higher rate to generate the energy that keeps them from collapsing under their own weight. Some low-mass stars will shine for trillions of years – longer than the universe has currently existed – while some massive stars will live for only a few million years.
The Sun is a class G star; these are yellow, with surface temperatures of 5,000–6,000 K. Class K stars are yellow to orange, at about 3,500–5,000 K, and M stars are red, at about 3,000 K, with titanium oxide prominent in their spectra.
Alpha Centauri C lies measurably closer to us than the other two: It is only 4.22 light-years away, and it is the nearest individual star to the Sun. Because of this proximity, Alpha Centauri C is also called Proxima (Centauri).
M stars are defined as a class of unevolved, zero-age stars that can be characterized by their mass distribution as described by the Initial Mass Function (IMF), which relates the number of stars to their mass intervals and luminosity.
As stars grow older, their luminosity increases at an appreciable rate. Given the mass of the star, one can use this rate of increase in luminosity in order to determine the age of the star.
Whatever the reason, it is extremely unlikely that we will ever observe a Pop III star and they will remain hypothetical entities.
With a redshift of z = 8.2, at the time of observation, the burst was the most distant known object of any kind with a spectroscopic redshift. GRB 090423 was also the oldest known object in the Universe, apart from the Methuselah star. The light from the burst took approximately 13 billion years to reach Earth.
Prof Haley Gomez, Head of Cardiff University's School of Physics and Astronomy, said: "Sometimes called red and dead or zombie galaxies, ellipticals are different to spirals like the Milky Way - they are made up of swarms of stars and they do not have core-collapse explosions - and yet Cardiff astronomers discovered a ...
Leaving the galaxy far enough to photograph it is a whole different undertaking for a species that has not yet left the Solar System. "To get [images of the Milky Way] a spacecraft would have to travel either up or down from the disk of the Milky Way, and travel so incredibly far," Doten explains.
Subsequent clashes over 2 billion years will give rise to a combined elliptical galaxy, replete with stars scattered in new orbits. It seems Earth, the sun and planets in our solar system will survive the crash but take on new coordinates in the cosmos.
The Andromeda Galaxy, 2.5 million light years away, is the most distant object easily seen by the unaided eye. Other denizens of the night sky, like stars, clusters, and nebulae, are typically hundreds to thousands of light-years distant.
Stars die and vanish from sight all the time, but astronomers were puzzled when one that had been stable for more than a decade almost disappeared for eight months. Between late 2024 and early 2025, one star in our galaxy, dubbed ASASSN-24fw, dimmed in brightness by about 97%, before brightening again.
A star can remain a red giant until the remaining supply of helium in its core runs out. Then it transitions into the final stage of its life cycle. Depending on the size and solar mass of the star, its life will end in one of two ways: as a white dwarf star or as a supernova explosion.
Interestingly, red dwarfs can have lifespans ranging from billions to trillions of years, far exceeding that of the Sun, and they exhibit stellar activity similar to the Sun, including flares and spots.
The classes are called O, B, A, F, G, K and M. Stars in the 'O' class are the most massive and hottest, stars in the 'M' class are the smallest and coolest.