Newly Analyzed Star is One of Ten Largest Ever Discovered

By Bill Andrews | March 12, 2014 6:00 am
Yellow hypergiant star HR 5171 and its companion are so close they actually touch, as in this illustration. // Credit:  ESO

Yellow hypergiant star HR 5171A and its companion are so close they actually touch, as in this illustration. Credit: ESO

You might think that with all the crazy phenomena in space —  black holes and rogue planets and expanding space-time — at least the humble star might be something we fully comprehend. But you’d be wrong. A star is about as simple to understand as a cell; sure we know a lot, but there’s so many stars, and so many varieties, that we might never know it all.

And that’s exactly the case with a star we’ve known of for years, a yellow hypergiant called HR 5171A. New analysis indicates that this star is the largest of its type yet found — and one of the ten largest stars known. Studying it has shed new light on how these rare giants behave.

Yellow Hypergiant Journalism

Though it conjures images of an enormous Homer Simpson after too much sugar, a yellow hypergiant is just another kind of star, like our sun or a red giant or a white dwarf. What makes yellow hypergiants special — other than their enormous size and their pale-yellowish color — is their rarity. We only know of 14 such stars in our galaxy, and they’re likely a brief phase that huge stars pass through during their lifetimes.

Photo showing HR 5171A

HR5171A is about 12,000 light-years away, but it’s so huge and bright that it’s visible to the naked eye (it’s just below the center in this image). Credit: ESO/Digitized Sky Survey 2

And HR 5171A (here’s its stats, for you observationists out there) is particularly hypergigantic. As an upcoming paper in Astronomy & Astrophysics details, the star is about 1300 times as wide as the Sun, or around 12 times the average Earth-sun distance. That’s big enough to put it among the top ten largest stars we know about.

It’s also about a million times brighter than our sun. HR 5171A lies about 12,000 light-years away, in the constellation of Centaurus, but it’s bright enough that you could still spot it with the naked eye under ideal conditions.

Lives of the Stars

But perhaps the wildest thing about this star is its companion star. This smaller sun orbits HR 5171A every 1300 days or so, and their surface temperatures are both around 5000 K. And, oh by the way, the companion is so close that the stars are actually touching. The press release even quotes the lead author as saying, “The whole system resembles a gigantic peanut.” Quite the mental image.

And the research team didn’t just rely on current observations for insights into this system. They also examined over six decades’ worth of data to determine that HR 5171A has grown larger and cooler over the past 40 years, suggesting it will soon transition to another kind of star (a blue variable or Wolf-Rayet, for example). They were lucky to capture it during this brief point in its evolution.

Learning more about this temporary class of star should help astronomers not just understand the life cycles of the biggest stars, but also how all stars change and interact with companions. Just one more step on the road to finally knowing all there is to know about stars.

CATEGORIZED UNDER: Space & Physics, top posts
  • amphiox

    If that picture is accurate, then the “smaller” star is still ginormous….

  • MeriMakr8298

    I’m having issues trying to figure out how the smaller stellar object isn’t just either absorbed or stripped of its mass to the point it would cease to exist. This thing is perplexing to say the least.

    • EverSubtle

      What if …. the small companion was once the parent star, larger even then HR 5171A ?
      Maybe it’s in a slow absorption.

      • MeriMakr8298

        Ah, interesting thought. not feasible from a physics point of view. But the image isn’t really accurate in any way, so I see how you might get that notion.

        I am not saying I don’t buy the explanation they give, I just find it sort of fantastical.

        Thank you for your reply.

        • EverSubtle

          Another theory for you if I can indulge.
          The companion for unknown reasons, has the same mass.

          • MeriMakr8298

            That sounds interesting, but if it was the same mass (which admittedly makes a lot of sense) it would have to be made out of something that wouldn’t show up as yellow. Yellow stars are a specific composition/density.


      The smaller star is being stripped. The process is continuing until a new phse come into play whereby the smaller star does eventually get absorbed by the hypergiant.

      • MeriMakr8298

        But if they’re TOUCHING it would stand to reason it shouldn’t take long at all for the smaller object to be absorbed – the thing is gigantic almost imagination. Even if the secondary sphere is a couple times the size of our sun, it doesn’t seem like it would last very long at all.


          The surfaces have not yet touched, but gas from the smaller star is feeding the giant. As the orbit of the smaller star decays, however, it will finally combine by the larger star.

  • Pankaj Saksena

    If the other star is so close then won’t the larger star’s gravity rip it apart? Won’t it break apart and collapse into the bigger one?

    Weird to say the least!

    • MeriMakr8298

      I cannot work it out either. It makes zero sense.

  • Uncle Al

    Is HR 5171A’s rotation axis pointing at us? If so, it is a whole world of hurt – our world – when it goes core collapse supernova.

    • MeriMakr8298

      12,000 light years is plenty of room, even if it was ‘aimed’ at us in the case of a supernova. Only issue might be if a neutron star was discharged in the explosion and then was aimed stright at us. But that would take THOUSANDS of years to get here and we would have ample warning.

      • Longmire

        It very well could have happened 12,000 years ago and we wouldn’t know it till to late.

        • MeriMakr8298

          Ah, no. Light travels faster than neutron stars. We would see it coming.

          • MeriMakr8298

            Maximum speed of a neutron star is 400 k/s (kilometers per second) where light is going 299,792.5 k/s That’s a HUGE difference of scale and speed.

  • Bobareeno

    This is nothing more than a “living” Mandelbrot fractal taken out to its near infinite extent. Or else a bra for a non symmetrical goddess

  • Jimmy Rustles

    The only thing you morons fail to realize is that the companion star’s momentum is far greater than the gravitational pull of the larger star, which suggest that the companion star was captured, traveling at a speed that you nit wits could never imagine was possible because you were spoon fed disgusting state funded education.


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