Guide to the Cosmos

 Making the Wonders of our Universe Accessible to Everyoneo.

Missed the Exploding Star? Watch the Rerun!


Eta Carinae is one of our Milky Way galaxy’s most massive, most luminous, and most bizarre stars. Well, actually, it is a binary system of two very massive stars; one contains over 100 times, and the other about 30 times, our Sun’s mass. These two stars release 5 million times more light energy than our Sun, and are  one of the 20 most massive stars in our galaxy. Below is a recent Hubble space telescope image of the environs of Eta Carinae. What we see is a vast nebula, 20 times more massive than our Sun, spanning about 3 trillion miles. All this is gas ejected by the massive star that lies hidden within.



From 1837 to 1858, Eta Carinae exploded spectacularly, peaking in 1843 to become the second brightest star in our night sky after Sirius. (Sirius seemed brighter only because it is about 900 times closer to Earth.) Unfortunately, this remarkable event occurred before cameras and modern telescopes existed; we have only notes taken by astronomers who used the much smaller telescopes of that period.


If you happened to miss the action 170 years ago, don’t worry. We can now seeing cosmic reruns due to “light echoes.” The image below illustrates how this is possible.



Eta Carinae exploded in about 5660 BC, at a distance of 7500 light-years from Earth. Recall that one light-year is the distance that light travels in one year. Hence, light from that explosion took 7500 years to reach Earth, arriving 170 years ago. But the light spread out in all directions. Some of that light hit a large dust cloud and was reflected, and some of that reflected light headed toward Earth. Astronomers call the reflected light a “light echo.” The reflected light took a longer path to reach us, traveling first to the dust cloud and then onward to Earth, It thus took more time to reach us than did the direct light beam. In fact, the path of the reflected light was 1000 trillion miles longer so it took an extra 170 years to get here, arriving just recently.


The left side of the image below shows the Carinae Nebula, with Eta Carinae near the top and the position of the dust cloud in the boxed area below. On the right side are three close-up pictures of the dust cloud taken on: March 10, 2003; May 10, 2010; and Feb. 6, 2011. The dust cloud images show progressive brightening as the explosion intensified 7670 years ago. We are now watching (for the second time) the explosion proceeding as if in real time. In reality, what we see is delayed by the travel time of light across vast cosmic distances. We are seeing the past unfolding in our telescopes.



The Eta Carinae explosion was quite unusual in that it extended over 20 years and was not a true supernova, as one might expect from such a massive star. Indeed, the changes in light intensity that astronomers now observe in the reflected light match the light intensity changes recorded by astronomers who observed the original blast 170 years ago. Based on that, we can expect more big “burps”.


In the longer term, Eta Carinae is a prime candidate to become a supernova or even a hypernova, explosions that would make the 1840’s event look like a hiccup. That prior explosion is just the rumbling of an unstable, immense star in the death throes, preceding its final catastrophic explosion. Astronomers expect that final explosion to occur anytime between next Tuesday and a million years from now.


We haven’t had a good supernova in our galaxy for 400 years. We are long over due, but it might be nicer if it were a bit farther away. At 7500 light-years, life on Earth will probably not be imperiled. The greatest danger at this distance would be from a gamma ray burst, but these narrow beams of immense energy typically shoot out along the star’s axis of rotation. Some astronomers claim Eta Carinae’s axis is not pointed in our direction. Hopefully they’re right.


Best Regards,
March 26, 2013
Note: Previous newsletters can be found on my website




Dr. Robert L. Piccioni


Author of
"Everyone's Guide to Atoms, Einstein & the Universe"
"Can Life be Merely an Accident?"
"A World Without Einstein"
Guide to the Cosmos



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