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Friday, 6 April 2018

10 WEIRDEST STARS IN THE MILKY WAY

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The 10 Weirdest Stars in the Milky Way
By John Wenz,
Popular Mechanics, 3 April 2018.

Carl Sagan once called the Sun an "ordinary, even a mediocre star." Most of the hundred billion or more stars in the Milky Way are humdrum and plain, but over the years astronomers have found plenty of peculiarities. In a universe bound only by the laws of physics, sometimes stars transgress the ordinary and get weird.

1. "My Favorite Enigma": Przybylski's Star

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Credit: Johannes Hevelius via Barry Lawrence Ruderman Antique Maps Inc.

Within the Centaurus constellation you'll find the Alpha Centauri group, three stars that make up our closest neighboring star system. But also hiding inside this southern sky constellation, just beyond what our eyes can see, is a truly weird star.

Przybylski's Star, or HD 101065, is about four times the mass of the sun. Stars are fusion engines, creating heavier elements inside their cauldrons, but Przybylski's Star is cooking up the kinds of elements we don't normally see in stars - including uranium. (The Sun, like many stars, doesn't typically fuse anything heavier than nickel, which is number 28 on the periodic table.) In fact, this star seems to form only heavier elements, the kind of stuff seen in violent events like supernovae.

Jason Wright, an astrophysicist who also studies astrobiology at Penn State, called the star his "favorite astrophysical enigma" in a blog post. He says most of these elements have short half-lives, so something may be replenishing them. Searches for a small pulsar have failed, but it could be that the star cooked up super heavy elements that are decaying into ones never found on Earth.

Or maybe it's aliens dumping their trash into the star. (We just have to move past every other possible solution first.)

2. The One That Made People Think It Was Aliens: Tabby's Star

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Speaking of it's-not-aliens, let's talk about Tabby's Star.

The planet-hunting Kepler spacecraft turned up a plethora of new worlds by looking for the telltale dip in a star's light that indicates a planet passing in front of its star. Most of the time we're talking about a 1 percent dip in light. That's why a group of amateur planet hunters were so puzzled by one object Kepler saw. Officially named KIC 8462852, the star is 1,200 light years away in the constellation Cygnus, and is about 1.4 times the mass of the Sun.

What intrigued astronomers about this otherwise ordinary star is that a transiting object caused it to dip in light by as much as 20 percent. That object had to be something far, far bigger than a planet. A group of professional planet hunters - including Wright and led by Tabetha S. Boyajian - came up with a number of scenarios as to what might be going on. It included a swarm of comets or - again, as a last resort - aliens. The idea was that something causing that deep a transit might be an artificial structure, including a theorized "Dyson Swarm" of object harnessing power from the star.

Subsequent observations have likely ruled out anything artificial. Instead, it seems to be a debris field of dust, though there are various arguments as to the nature of the dust. (Disintegrating comets or a destroyed planet have been brought up.)

Astronomers haven't witnessed anything like the original detection since, though a series of observations recently detected five percent dips in light, which is still far too large to be a planet. But investigators are continuing to try to get to the bottom of the mystery.

3. Zombies and Diamond Planets: PSR J1719-1438b

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Neutron stars are weird to begin with. They are the dense remains of supernovas made mostly of neutron matter. A neutron star packs the mass of the Sun within the size of a small city. Some neutron stars are pulsars, meaning their core rotates in a way that gives out a consistent radio source. And pulsar planets are already weird. They form after the supernova, giving the star a second generation of zombie planets.

The weirdest pulsar-planet combination we know about has to be PSR J1719-1438 and its planet, which may not have started out as a planet at all. The object is a dense hunk of carbon that’s been called a “diamond planet” due to the pressures it was formed under. In all probability, though, it didn’t start out a planet. Instead, it was a star locked in a binary orbit with the larger star that exploded. Once the larger star went supernova, it likely stripped out the gasses around the smaller star and condensed it, eventually pulling it into a 10 hour orbit.

As it turns out that this planet may be a bizarre dead star. Or perhaps something even weirder - an even smaller hunk of ultra dense quark matter created from when two neutron star-like objects called quark stars (made out of quarks rather than neutrons) merged and annihilated each other.

Whatever it is, PSR J1719-1438b is not a planet by any typical definition of the word.

4. Older Than the Entire Universe? HD 140283

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The universe is 13.8 billion years old. HD 140283, by all appearances, is 14.4 billion years old. That is a problem.

The object, informally called "Methuselah's star," also has all the chemical signatures you'd expect from a second-generation star (one formed from gas and dust after the first generation of stars exploded). So now things are really weird.

The age estimate is based on its composition, brightness, and distance (the star is 190 light years away.) Its minimum estimated age is 13.2 billion years, and the margin of error of the 14.4 billion year estimate probably places it comfortably - but barely - within the age of the universe. So even if it's not as old as the universe, it's still far and away the oldest star near to us.

5. The Speed Demon: HE 0437-5439

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Most stars have fixed orbits around the center of our galaxy. But HE 0437-5439 is getting the hell out of here at 1,600,000 mph.

The 30-million-year-old star was likely formed near the center of the Milky Way. An encounter with the supermassive black hole at the center of our galaxy, Sagittarius A, probably threw it out of the galaxy and into interstellar space.

There are a few discrepancies with its trajectory - for instance, given its position right now, it seems to have travelled too far for its age. It's possible that it was actually a triple star that got too close to Sagitarrius A, got flung out, and this interaction also destabilized the three until they merged into one younger, brighter star.

6. The Star That's Tearing Itself Apart: Vega

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Credit: sebastien lebrigand/Wikimedia Commons

Vega is a big, bright, young, nearby star that is seemingly normal - save for a few hiccups in that assessment. For one thing: The star is brighter than it should be.

The deeper you go, the weirder Vega gets. For instance, it doesn't seem to be round so much as ovular. And astronomers think they know why: it's rotating so quickly that the star's shape is distorting, even causing variations in temperature across its surface. Its equator is 23 percent larger than its poles, and it is going 93 percent of the maximum velocity a star its size can handle at a blazing 170 miles per second. This may be even throwing off age estimates by as much as 200 million years.

Astronomers are still working on figuring out why Vega is going so fast its nearly tearing itself apart. Even some of our nearest neighbors in space are hiding secrets from us.

7. The Star Inception: HV 2112

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What if you put a star inside another star? You might end up with something like HV 2112.

In 1977, physicists Kip Thorne and Anna ┼╗ytkow came up with a theoretical class of stars called a Thorne–┼╗ytkow object (TZO). Such a phenomenon would be formed when a neutron star cannibalizes another star in a hit and run.

To a faraway observer, it would appear to be a red giant star, but a little bit...off. A TZO would grow huge - possibly encompassing an area the size of Saturn's orbit. The dense core - a merger between the neutron star and the core of the main sequence star - transports heavy elements like lithium, molybdenum and rubidium up into the upper gas envelope of the star, giving off strange spectrum.

HV 2112 (which is either in the Milky Way or in our neighbor dwarf galaxy, the Small Magellanic Cloud) seems to have all these hallmarks, which would make it the first TZO ever discovered.

8. The One With a Tail: Mira

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Mira can sometimes be one of the fainter stars in Cetus. Other times, it's one of the brightest stars in the sky.

The system is actually two stars. One is a red giant that was once like the Sun, but now has expanded outward as it begins its death spiral. The other is a white dwarf - the stage of a sun-like star after it's become a red giant.

As the red giant in the system is shedding its outer layers, it may be creating clouds of debris around the close-knit system. This could be obscuring the stars until the envelope expands enough to let the stars shine again - which also explains why the intervals aren't regular as in most variable stars. It also gives the stars a comet-like tail.

9. The Ultra-Cool Dwarf: EBLM J0555-57Ab

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Recent advancements in infrared sensors and other telescope optics have given astronomers new insight into the smallest stars in our galaxy.

These stars are called ultra-cool dwarfs, with TRAPPIST-1 being perhaps the most well known example. It's a Jupiter-sized star that still gives off heat and light by fuzing hydrogen into helium, and is 8.9% the mass of the Sun.

But the ultracool dwarf EBLM J0555-57Ab may stretch the boundary between a star and a brown dwarf - and is thus far the smallest main sequence star ever discovered. It's about 8.1 percent the mass of the Sun, but has the radius of Saturn. It even orbits its parent star, EBLM J0555-57A, like a planet in a matter of 7.8 days.

10. The Largest Star in the Galaxy: UY Scuti

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Credit: Haktarfone/Wikimedia Commons

Imagine, if you will, a star that's the size of our entire inner solar system, that even engulfs the radius between the Sun and Jupiter, where a photon from the core would take 43 minutes to reach the surface if nothing got in its way. Then you'd have UY Scuti.

The volume of the star is 5 billion Suns, meaning you could fit in every object in our solar system inside the star many times over and still have plenty of room to spare. It is thus far the largest - though not most massive - star known in our galaxy. (The most massive star in our galaxy is about 120x the mass of the Sun, whereas UY Scuti is seemingly only 7-10 solar masses, meaning it's large, but not necessarily dense.)

Top image: The Pleiades star cluster, also known as "The Seven Sisters." Credit: NASA, ESA, AURA/Caltech, Palomar Observatory/Wikimedia Commons.

[Source: Popular Mechanics. Some links added.]

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