Cassini: the end of a legend

20 years after it launched, the NASA spacecraft Cassini ends its mission today. Its 13 years in orbit culminate in one last mission: to dive into the planet’s atmosphere, while still measuring and transmitting data.

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The heavens above Mauna Kea – Tales from Hawaii: Part 2

Mauna Kea is an enormous, exinct volcano on Hawaii’s Big Island, and one of the best places on Earth to do astronomy (if you ask them, the best one). In fact, the summit of the mountain hosts one of the world’s most renowned observatories. What makes it so special, other than that it looks like this?

Three of the telescopes on Mauna Kea’s summit. In the background, beyond the clouds is Maui (more than 100km away). CC-BY-NC-SA Carmen Romano

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A star like any other?

Think about it: the idea that the Sun is essentially the same thing as any star doesn’t make any sense. I mean, just look at them, they could hardly be more different!

Seriamente. Vi sembrano la stessa cosa? credit: CC-BY-ND Jessie Hodge via flickr

Seriously. Do they look anything like the Sun? credit: CC-BY-ND Jessie Hodge via flickr

So how the heck do you go about proving such an outrageous idea?

Well, it’s been quite a long journey,  that started from a number of guesses by ancient “scientists”. Granted, some of them turned out relatively correct, but others involved fiery stones hanging from the sky.

The first actually scientific step came only in 1838, when we learned how freaking far way stars are. German astronomer (among other things) Friedrich Bessel measured the distance to a star—now known as 61 Cygni—for the first time, without assuming anything about what the star was like. It turned out to be thousands of time farther than the Sun. Hundreds of thousands of times.

If they’re so far away, people concluded, stars might actually be as big as the Sun, if not bigger (spoiler: they get a lot bigger). But are they the same thing?

Shortly thereafter, we learned to read the chemical composition of a star from its light. Each element inside it, we learned, absorb light in a specific way. So when we look at the light from the star through a prism, we can see the thin black bands the elements leave behind and reverse-engineer what they were.


Solar light, broken down in its component colors, with the black bands (also called “Fraunhofer lines”) left by elements inside it. Via wikimedia

Then, we figured how to relate the colors (more exactly, the wavelength) of starlight to their temperature. As the video below explains, everything with a temperature—aka everything—glows, and it does in a particular way, depending on how hot it is. Only quantum mechanics explained how and why, but it works: it’s how infrared thermometers operate.

Altogether the Sun turned out about average in size, temperature and composition. Yet, it was a special star: the only one we knew to have planets. That, too, changed. It took a while (until the 1980s), but nowadays we find exoplanets—planets orbiting other stars—by the thousands.

So the Sun is just a star. We’ve thought about the universe and literally just looked at it, and we understood something so violently counterintuitive. No star that we know is home to anything with such remarkable ability. And that, I think, is pretty special.


Cover photo: CC0 Mayur Gala, via

If you want more
  • Nobody measured the distance to a star before Bessel because, at the time, it was really hard. Here’s a wikipedia page on the ingenious method he used.
  • Actually, quantum mechanics was born trying to explain how things glow depending on their temperature. This video from PhysicsGirl explains how