Identical Twin Stars Not So Identical

Article written: 18 Jun , 2008
Updated: 24 Dec , 2015

I’m lucky enough to have twin sons. They aren’t identical (one looks like me, the other looks like my husband – which is about as different as things get) but they have a lot of similarities. One of my favorite stories about having twins is the time we took the whole family out to a restaurant shortly after the twins were born. The waitress commented that our babies looked the same size, and we said, “Yes, they’re twins.” And she replied, “Oh really? How far apart in age are they?”

I used to think that waitress was a real ditz, but after seeing a press release today from Vanderbilt University, I’m wondering if the waitress was on to something, and maybe she was even an astronomer.

Astronomers recently found a very young pair of identical binary stars that have surprising differences in brightness, surface temperature and size. They also believe one of the stars formed significantly earlier than its twin. Astrophysicists have assumed that binary stars form simultaneously, and so this discovery forces theorists back to the drawing board to determine if their models can produce binaries with stars that form at different times.

The identical twins were discovered in the Orion Nebula, a well-known stellar nursery, 1,500 light years from Earth. The newly formed stars are about 1 million years old. With a full lifespan of about 50 billion years, that makes them equivalent to one-day-old human babies.

“Very young eclipsing binaries like this are the Rosetta stones that tell us about the life history of newly formed stars,” says Keivan Stassun, associate professor of astronomy at Vanderbilt University. He and Robert D. Mathieu from the University of Wisconsin-Madison headed up the project.

The astronomers calculated that these twin stars have nearly identical masses, about 41 percent that of the sun. According to current theories, mass and composition are the two factors that determine a star’s physical characteristics and dictate its entire life cycle. Because the two stars condensed from the same cloud of gas and dust they should have the same composition. And with identical mass and composition, they should be identical in every way. So the astronomers were surprised when they discovered that the twins exhibited significant differences in brightness, surface temperature and possibly size.

“The easiest way to explain these differences is if one star was formed about 500,000 years before its twin,” says Stassun. “That is equivalent to a human birth-order difference of about half of a day.”

Now, I have heard stories of twins being born several hours apart and even in different years (one late on Dec. 31, and the other early on Jan. 1) so, maybe this difference in star formation isn’t such a big deal, and it happens all the time. However, further study is needed.

But this new discovery may cause astronomers to readjust their estimates of the masses and ages of thousands of young stars less than a few million years old, as current estimates are based on models that presumed binary stars formed simultaneously.

Just like having twins causes you to readjust your entire life. But it’s a good readjustment.

Original News Source: Vanderbilt University (this link includes a nice multimedia presentation about the discovery)

13 Responses

  1. Dutch Delight says

    I found the article interesting but I’m a bit confused about what actually was discovered here.

    “Astrophysicists have assumed that binary stars form simultaneously, and so this discovery forces theorists back to the drawing board to determine if their models can produce binaries with stars that form at different times.”

    This can’t be right, it gives a wrong idea on how astronomy is done as well. I’m pretty sure people were making models for both eventualities and at most you could say that the distribution of these different types of binaries is different from what was expected.

    I’m not aware of any hypothesis that says that stars can’t capture each other gravitationally when the oppertunity comes along at pretty much any stage of their lifecycle. Of course, i might just be wrong since i don’t get paid to know this stuff.

  2. Excalibur says

    1 way to create equal mass pair with different properties – a tripple system, where 2 lower mass stars merge nearing the end of formation. Uncommon probably, but possible…

  3. alan says

    Another way is to have a very close approach between two sets of binary stars, or a single star and a binary star. With the right alignment they can switch partners.

  4. themadlolscientist says

    The “twins of different ages” thing used to happen a lot more in the days before the current “twins = automatic C-section” practice. One of my great-grandmothers gave birth to no less than three sets of twins the regular way (OUCH!), one pair of whom did have different birthdays.

    But I’m sure I don’t have to tell you what a big deal the “who’s older” thing is with twins regardless of the difference. Even a couple of minutes is enough for one to say proudly, “I’m the oldest!” (and probably get a poke in the ribs from the other). 🙂

  5. Steve says

    I still don’t see why scientists would speculate that all binaries are identical.
    If all gas clouds had a uniform density of material, then yes they would be identical.

    The question I would like to know is have we ever found a perfectly uniform distribution of gas? If we haven’t, Im wondering what trash can they pulled this theory out of.

    Im no rocket scientist, but one with even limited knowledge on the subject would assume that a star would form much faster in a more dense part of a gas cloud and grow at a much faster rate than its “twin” if it forms in a less dense area

  6. Cap says

    Baby twins you say, cant wait to see them grownup and stealing each other’s gas 😉
    I heard it said that stars are made of cold thick clouds of gas and dust and if a star happens to be nearby the now energetic particles will disperse rather then collates necessary for the very hot core and the first nuclear Fusion, hence the need for twins to explain a binary star system, it is possible 500k years is not enough time for star to clears its neighborhood

  7. Astrofiend says

    Dutch Delight Says:
    June 18th, 2008 at 5:45 pm
    “I’m not aware of any hypothesis that says that stars can’t capture each other gravitationally when the oppertunity comes along at pretty much any stage of their lifecycle. Of course, i might just be wrong since i don’t get paid to know this stuff.”

    Stars can capture each other, but only if there is three or more stars involved in the interaction to remove energy from the two-star system in question. Conservation of energy precludes two stars forming a binary system on their own if they were not formed together. Now, although two stars are never truly in isolation, three (or more) stars have to interact in a fairly specific way to produce a binary system. This makes such an event highly unlikely – it probably does happen, but only for a vanishingly small number of stars.

    Steve Says:
    June 18th, 2008 at 10:38 pm

    “I still don’t see why scientists would speculate that all binaries are identical.”

    Scientists do not speculate that all binary systems are identical – it is an observational fact that many are not. But the story states that it is a young pair, and it also happens to be an observational fact that pretty much all young binary systems consist of very similar members. Accordingly, formation theories have taken this fact into account and aimed to explain it. Until there was observational evidence to suggest otherwise, it would be nothing but speculation to suggest that young binary members may be dissimilar!

    And now there is observational evidence to suggest that binary members may indeed be dissimilar in age and mass, so obviously a mechanism for breaking this symmetry will have to be found and incorporated into the current theory. Which doesn’t make the current theory trash, it just expands upon it and refines it where required as suggested by the evidence at hand.

  8. Aodhhan says

    What if we think about the formation of a star to be identical to the way a pearl or synthetic diamond, where there is the initial ‘seed’. The quality of the bonds within the seed can change the outcome of the entire product.

    Suppose the ‘seed’ to these stars are slightly different. One is initially able to gravitate gas and material quicker than the other.

    Then add the fact it is possible one star was formed in a slightly denser column of gas than the other; allowing it to gain critical mass sooner than its sibling.

    Certainly, stellar nurseries aren’t constant in their density of material and gas; so it is possilble stars located close together could be different than the other. However, I believe this is probably more the exception than the rule.

  9. Bill says

    Does anyone notice that there is not much predictive ability in stellar formation models? The answer here is to go back and tweak the model (computer code) until it matches what’s seen and then declare success. There needs to be falsifiable criteria for the models; we never see that – it’s a forgotten tenet of the scientific method. At what point do we scrap the theory (the accretion model)? Remember, there is still no account for 99% of the angular momentum that has to be dissipated in the standard accretion model. That’s a big hole for the theory.

  10. Dutch Delight says

    Thanks Astrofiend!

    Bill, observing stuff IS a method of falsifying models, it’s done all the time.

  11. Bill says

    DD, Falsification is a set of criteria, in advance of observation and data analysis, that a researcher says will falsify her theory if shown to exist.

  12. lrholliday says

    How do they know that the two stars have identical masses? I know that the combined mass can be deduced from the orbits, but how do they deduce that the combined mass is evenly divided? Since one of the two stars is brighter and hotter than the other, I would be tempted to say that it has a larger mass.

  13. Dutch Delight says


    You are a troll. First you post a rant where you pretend that stellar formation is all a big mystery (even though we’ve got libraries with observational data on all kinds of stars in pretty much every stage of their development) and now you are apparently arguing that the models that scientists make of reality cannot be falsified by… observing reality!?

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