Unseen Cosmos

Extreme stars

Eta Carinae

A pair of giant stars that survived an outburst bright enough to rival Sirius

Artist's impression · drag to rotate
Stars 2 orbiting each other every 5.54 years [1]
Distance about 7 500 light-years [1]
Primary star about 100 × the Sun's mass [1]
Secondary star 30–80 × the Sun's mass [1]
Luminosity more than 5 million × the Sun [1]
Great Eruption 1837–1858 second-brightest star in the sky in 1843 [1]
Mass thrown off at least 10–20 × the Sun now the Homunculus Nebula [1]
Brightness today magnitude about 4 [1]

About the image. Drawn after Hubble's images of the Homunculus Nebula: two glowing lobes of gas blasted out in opposite directions from the bright central stars, with a thin skirt between them. The two stars cannot be separated by any telescope; their appearance is imagined.

In 1677 Edmond Halley recorded a fourth-magnitude star in the southern constellation Carina. In December 1837 John Herschel, observing from South Africa, noticed it had become much brighter. Over the following years it kept flaring, and in March 1843 it outshone Canopus to become the second-brightest star in the night sky, after Sirius. Then it faded until it could no longer be seen with the naked eye. This was the Great Eruption of Eta Carinae.

What erupted

Eta Carinae is not one star but two. The larger has about 100 times the Sun’s mass and may have started with 150 or more. Its companion has 30 to 80. They circle each other every 5.54 years on a very stretched orbit, and together they shine more than five million times as brightly as the Sun.

During the Great Eruption the system threw off at least 10 to 20 solar masses of gas, yet survived. That material now forms the Homunculus Nebula, the “little man”: two bulging lobes expanding in opposite directions with a thin, flat skirt between them, which Hubble has photographed in remarkable detail. A smaller outburst, the Lesser Eruption, followed in the 1890s.

Light from the 1840s

Nobody in the 1840s could take a spectrum. But some of the light from the eruption bounced off distant dust clouds and is only now reaching Earth. In 2012 Armin Rest and colleagues caught these light echoes and analysed them. They showed that, at its peak, the eruption had a surprisingly cool temperature of about 5 000 K (4 730 °C), cooler than models had predicted. Exactly what caused the eruption is still debated.

What comes next

Eta Carinae is expected to end as a supernova, possibly a very luminous one or a hypernova, leaving a black hole behind. At 7 500 light-years it would be a spectacular sight, perhaps about as bright as Venus, but too far away to harm Earth.

How long to get there?

Eta Carinae is about 7 500 light-years away.

  1. Light299 792 km/s 7 500 years
  2. Breakthrough Starshotplanned light sail, 20% of light speed 37 500 years
  3. Parker Solar Probefastest spacecraft so far, 192 km/s 11.7 million years
  4. Voyager 1leaving the Solar System, 17 km/s 132.3 million years
At Voyager 1's speed the trip would take about 1 653 265 human lifetimes of 80 years. Bars are on a logarithmic scale. Speeds: NASA, Breakthrough Initiatives.

Gallery

A brilliant white star between two large glowing orange lobes of gas that bulge out on opposite sides like an hourglass, an impression of Eta Carinae and the Homunculus Nebula. AI-generated painting.
A brilliant blue-white star shining from a hollow in billowing orange clouds of gas and dark dust, an impression of Eta Carinae in the Carina Nebula. AI-generated painting.
Eta Carinae, 3D artist's impression, rendered from the interactive model.
Eta Carinae, 3D artist's impression, from another angle, rendered from the interactive model.
Eta Carinae, 3D artist's impression, close-up, rendered from the interactive model.

Timeline

  1. 1677 Edmond Halley records it at fourth magnitude.
  2. 1837 John Herschel sees it brighten; the Great Eruption begins.
  3. 1843 It becomes the second-brightest star in the night sky.
  4. 1890s The Lesser Eruption.
  5. 2012 Light echoes reveal the eruption's temperature.

Questions people ask

Is Eta Carinae going to explode?

Yes, eventually. It is expected to end as a supernova or hypernova, but nobody knows when.

Is Eta Carinae dangerous to Earth?

No. At about 7 500 light-years it is too far away to harm us.

What is the Homunculus Nebula?

The two-lobed cloud of gas thrown off in the Great Eruption of the 1840s, at least 10 times the Sun's mass.

Sources

  1. Wikipedia: Eta Carinae
  2. Rest et al. (2012), Light echoes reveal an unexpectedly cool η Carinae during its 19th-century Great Eruption, Nature 482
  3. NASA Planetary Fact Sheet (planet temperatures, orbits, Jupiter's gravity)
  4. IAU 2015 Resolution B3 (nominal solar values)
  5. NASA: Voyager mission
  6. NASA: Parker Solar Probe
  7. Breakthrough Starshot

Keep exploring

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