10 Most Extreme Stars in the Universe

UY Scuti
Image Credit: Comparison of UY Scuti to other stars (Sun, Sirius, Pollux, Arcturus) by S. Clester

Our Sun is often described as an “average yellow dwarf star”, but the truth of the matter is that there are no average stars. Stellar bodies come in a bewildering variety of masses, luminosities, radii, and temperatures within any given classification or type, from barely making it into any given classification to representing the most extreme example of their classification. While the items on this list represent these extremities, it must be remembered that complex issues exist around determining the “vital statistics” of stars.

For instance, to calculate a stars’ true diameter, its luminosity must be known to a high degree of accuracy. This is often very difficult to determine because many massive stars are frequently encased in dusty envelopes that absorb and/or scatter much of the star’s light. Similarly, determining a star’s mass depends largely on knowing (among many other things, such as its metallicity), its distance, which in some cases, is very difficult to determine or even impossible due to factors that are not clearly understood, as is the case with some stars in Orion’s Belt.

For these and other reasons, the stars on this list are merely the leading contenders for the title of “Most Extreme Star in My Category”, and issues like stellar variability over long periods, improved observational techniques, or other factors may unseat some of the stars on this list in the future. Nonetheless, the stars on this list represent current knowledge, so let us start with..

The Biggest Star – UY Scuti

UY Scuti
Image: Rutherfurd Observatory/ Columbia University

The image above shows UY Scuti, a pulsating variable red supergiant star (the red spot to the right and above centre), which is located about 9,500 light years away in the constellation Scutum. In terms of sheer bulk, UY Scuti is the leading contender for the biggest star title, having a radius that is estimated to be 1,708 times that of the Sun, equivalent to 750 million miles, or 7.94 astronomical units, giving it a volume 5 billion times that of the Sun. If UY Scuti were placed over the Sun, its photosphere would extend past the orbit of Jupiter.

Although there is a margin of error of about 190 solar radii in determining the true diameter of UY Scuti, it would still take a hypothetical space ship moving at the speed of light (c) eight hours to travel around the star at its maximum diameter. By way of comparison, a space ship moving at the speed of light needs only 14.5 seconds to travel once around the Sun.

The Most Massive Star– RMC 136a1

RMC 136a1
Image: ESO/VLT

The bright light at the center of this near infrared image is the star RMC 136a1, situated in R163, the innermost region of the large open cluster NGC 2070 (in the Tarantula Nebula) in the Large Magellanic Cloud. It is located about 163,000 light years away in the constellation of Dorado.

RMC 136a1 weighs in at a whopping 315 solar masses, but at only about 800,000 years old it is still on the main sequence fusing hydrogen into helium. However, the star is undergoing extreme mass loss through an energetic solar wind that is blowing material away at a speed of about 2,600 km/sec, which is stronger than the gravitational forces that holds the star together.

At its current stage of evolution, R136a1 is losing mass at the rate of 5.1 × 10−5 M (3.21 × 1018 kg/s) per year, which translates into about one billion times higher than the Sun is losing its mass. At this rate, R136a1 would have lost about 50 solar masses worth of material since its formation.

The Brightest Star – R136a1

During 2010, investigators discovered that R136a1 is also the most luminous star known. While previous estimates put the stars’ brightness at about 1.5 million solar luminosities, R136a1 is now known to shine with at least 8.7 million solar luminosities, which means that R136a1 emits as much energy in five seconds, than the Sun emits in a full year.

To put this in perspective- if R136a1 were to replace the Sun in the solar system, we would see it at magnitude -39, which is at least 94,000 times brighter than the Sun it would have replaced. In addition, from a distance of 10 parsecs away, its absolute magnitude would be -7.6, which is more than three times brighter than Venus ever gets when viewed from Earth.

The Smallest True Star – EBLM J0555-57Ab

EBLM J0555-57Ab s the smallest star in a triple-star system that is located about 600 light years away in the constellation Pictor. As the smallest star in the system, EBLM J0555-57Ab has a mass of about 85 times that of Jupiter (about 0.018% of that of the Sun), and a radius that compares to that of Saturn. With these numbers, EBLM J0555-57Ab represents the lower mass limit at which true stars can initiate and sustain a process of hydrogen fusion, according to current models of star formation.

The other two stars in the system are EBLM J0555-57Aa, a F-type star, and EBLM J0555-57B, a magnitude 10.76 star about which not much is known. Although no orbital motion has been detected in the system, all there stars share a common proper motion, which suggests that they are gravitationally bound.

The Hottest Star – WR 102

WR 102
Image: NASA/ESA/Hubble

With a surface temperature of 210,000K compared to 6,000K for the Sun, WR 102 (shown here illuminating the nebulosity that surrounds it) situated 9,800 light years away in the constellation Sagittarius is the hottest known star. It is also an oxygen-sequence Wolf-Rayet star, which shed their surplus mass extremely quickly, and are extremely rare, with only a few found per galaxy and four known in our Milky Way.

Its luminosity is estimated to be about 500,000 times that of the Sun, and while it has a diameter of less than half of that of the Sun, it is at least 20 times more massive. Based on its spectrum, WR 102 has reached, or is close to the end of its helium burning phase, and it is expected to self-destruct in a supernova explosion sometime in the next 1,500 years or so.

The Fastest Moving Star – HE 0437-5439

Located about 200,000 light years away in the constellation Dorado, HE 0437-5439 is a massive, B-type main sequence star with an estimated age of about 30 million years. HE 0437-5439 appears to be receding from Earth at the break-neck speed of 723 km/sec (449 miles/s), or 2.6 million km/hour. At this extremely high-speed, the star is moving beyond the escape velocity of the Milky Way galaxy, and will eventually move into intergalactic space. By way of comparison, most other stars travel at a more sedate pace of about 100 km/sec or so.

Note, though, that there exists another contender for the title of fastest moving star. The star designated RX J0822-4300 (now moving away from the Puppis A supernova remnant) has been measured to move at 1,500 km/s, or 5,400,000 km/h (3 million miles/hour). However, since no clear mechanism has been found to explain the high recessional speed of RX J0822-4300, its velocity has been revised downward to more modest 672±115 km/s, which is not quite so difficult to explain theoretically. In both cases, it is possible for stars to attain such high speeds when they are ejected from binary systems, or when they pass around massive black holes that fail to capture them.

The Fastest Rotating True Star – VFTS 102

While most neutron stars are known to have very high spin rates, with PSR J1748-2446ad known to spin at 716 times a second, or almost one-quarter the speed of light, neutron stars are not normal stars and therefore do not qualify for a place on this list.

The fastest spinning star in which fusion processes are taking place is VFTS 102, a 25-solar mass star located about 160,000 light years away in the Tarantula Nebula in the Large Magellanic Cloud in the constellation Dorado.. This star is spinning at roughly 2 million km/sec at its equator, which is about 300 times faster than the Sun, with this rate of rotation thought to represent the highest theoretical limit at which gaseous stars can spin before they break apart.

The Reddest Naked-eye Stars – Mu Cephei / CE Tauri

Mu Cephei and CE Tauri
Image: Greg Parker

The composite image above shows Herschel’s Garnet Star (Mu Cephei) in the constellation Cepheus, and the Ruby Star (CE Tauri) in the constellation Taurus in the top and bottom frames of the panel, respectively. While there are brighter red stars than these two, such as Betelgeuse, Arcturus and Aldebaran, there are no other naked-eye stars known that are redder.

It’s all in the numbers; Mu Cephei has a B-V index of +2.35, and an apparent magnitude of 4.08, which makes it both brighter and redder than CE Tauri. CE Tauri on the other hand, has a B-V index of +2.07, which makes it redder than Betelgeuse, Arcturus, and Aldebaran, since their B-V indexes are 1.85, 1.34, and 1.78, respectively. Note that the higher the positive value of the B-V index, the redder the star is.

The Oldest Star in the Milky Way – HE 1523-0901

HE 1523-0901The image above is an artists’ impression of “Methuselah”, the oldest known star in the Milky Way, located about 7,500 light years away in the constellation Libra. Methuselah is visible in small telescopes, but note that while it is best seen from the southern hemisphere, it can be seen up to mid-European latitudes, too.

Methuselah is an extremely metal-poor, 0.8-solar mass star that was found among other metal-poor stars in the galaxy’s extended halo, and is thought to have formed directly from the remains of a previous generation of stars, or Population I stars. Using the ESO’s Very Large Telescope, the star’s age was determined to be about 13.2 billion years, which is nearly as old as the Universe itself.

However, while the margin of error in the method used to determine its age (mainly through measuring the rate of decay of the elements uranium and thorium it the star) is between 0.7 and 2.7 billion years, depending upon the assumptions at play in determining the uncertainty, this figure reduces to about 800,000 years when the same method is used to determine Methuselah’s age relative to similarly ancient stars.

Note that since the uncertainties regarding the relative age of the Universe and Methuselah are both very large, the star is not older than the Universe that contains it. It is just nearly as ancient as the Universe.

The Most Distant Star – SDSS J122952.66+112227.8

IC 3418 Galaxy
Image: NASA

The image above shows the galaxy IC 3418 that contains the most distant star known called SDSS J122952.66+112227.8, located about 55 million light years away in the Virgo constellation. The galaxy is well-known for its long tidal tail that represents the tug-of-war between the galaxy and the Virgo Super Cluster of Galaxies, which IC 3418 had run headlong into about 55 million years ago.

The star SDSS J122952.66+112227.8 is an O-type, blue supergiant that is illuminating a clump of gas being stripped from the main galaxy, and as such, it is the furthest known star to be definitively resolved by means other than its effects on its surroundings. While many stars are known at greater distances from Earth, these stars can only be identified through the events they cause, such as supernova events and gamma ray bursts.

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