What is the Hottest Star?

This week, I’m writing about the sun. And about stars in general. Why? Because two days ago, while walking home from preschool, we started talking about the summer solstice and things escalated from there. Five year olds are fully capable of unleashing an avalanche of questions.

Yesterday, if you recall, I answered his question about why the sun didn’t melt. He accepted the explanation I gave him, but it led him to two more: “What is the hottest star? Is it the biggest?”

I was honest with im. I had no idea what the hottest star is, or if that star is the biggest star. Let’s find out the answer together, shall we?

Measuring Stars

Most commonly, stars are measured in “solar” or “stellar” units, based on the measure of our own star (aka “the sun”).

  • Solar Mass (M): the mass of the sun, which is 1.98855 x 1030 kilograms.
  • Solar Luminosity (L): the energy output of the sun, which is 3.828 x 1026 watts.
  • Solar Radius (R): the radius of the sun, which is 6.960 x 105 kilometers.

To put that in perspective, the Earth has a mass of 5.927 x 1024 kg and an equatorial radius of 6.378 x 103 kilometers. So that means that the sun has the mass of roughly a million earths, and is approximately 100 Earths wide.

None of these address temperature, though. The sun’s core is modeled to be 1.57 x 107 Kelvin (K), the photosphere is 5,772 K, and the corona is 5 x 106 K. (If you aren’t familiar, the kelvin is a measure of temperature; it’s identical to Celsius, except that 0 degrees kelvin is absolute zero, and water begins to melt at 273.15 kelvin.)

Harder. Better. Faster. Stronger.

So, what’s the brightest star? Based on luminosity it’s R136a1, located about 163,000 light years from Earth. It’s luminosity is 8,710,000 L, and it’s not a slouch in other matters as well. It’s the most massive star we know of as well, with an estimated 315 M. The radius, however, is “only” estimated at 28.8 to 35.4 R, so it’s a long way from being the largest star.

The honor of being the largest star belongs (right now) to Westerlund 1-26, which has a radius around 1,530 R. Although not the brightest or hottest, it does its best. Its luminosity is 380,000 L, but it’s photosphere temperature is a paltry 3,600 K – only 62% of the Sun’s and far cooler than R136a1’s 53,000 K.

R136a1 isn’t the hottest star going, though. That title belongs to H1504+65, which has an estimated photosphere temperature of 200,000 K – 34 times hotter than the sun.

Is there anything bigger?

Of course there is. There’s plenty of room for things to go big in space, after all. For example, there is a thing called a pair-instability supernova – technically a hypernova – that happens when a star with 130 to 250 M explodes. It generates 1011 L at peak output.

The single most massive, brightest thing we know of is S5 0014-81, “a distant, compact, hyperluminous, broad-absorption line quasar or blazar located near the high declination region of the constellation Cepheus”. It comes in with 40,000,000,000 M and about 300,000,000,000,000 L. To put that in perspective, if it was 280 light years away from us – about the distance to Theta Scorpii – it would give us as much energy as the Sun. Fortunatly, it’s more like 12,000,000,000 light years away.

Next to S5 0014-81, or Sagittarius A* at the heart of our own galaxy, or even our fairly ordinary home star, our Earth is a tiny speck in the universe.  But it’s home.

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