Tonight across the U.S. millions of us will be out watching the sky flare and boom with fireworks in honor of American independence. It’s also a rare night because we’ll all be doing the same thing at dusk — looking up at the night sky. Shining down from above, we’ll drop in on two stars that may soon add their own brand of firepower to the show.
Before the show, allow your gaze to stray south and see if you can pick out bright Mars, Saturn and the star Antares. Together they outline a longish triangle about a fist and a half wide. Then pivot 90° left and face east to spy Vega, the brightest and highest star in the even larger Summer Triangle that includes Deneb in the Northern Cross and Altair in Aquila the eagle.
We’re going to focus on Deneb and Antares, both supergiant stars but shining with different colors. Antares is an unmistakable Mars-like red, while Deneb shines a plain white. The super in supergiant isn’t to be taken lightly: Deneb’s 114 times larger than the sun and 15-16 times as massive. Antares is nearly 840 times solar and 15 to 18 times as massive. But at their respective distances of 1,425 and 550 light years, both stars appear as tame twinkly points of light in our night sky.
Both stars are so massive that they burn their nuclear fuel at an alarming rate, converting hydrogen in their cores to helium, helium to carbon and oxygen and on and on until iron is created. Each burning cycle fuses a heavier element from lighter elements and releases the energy needed to push back against the inexorable pull of gravity that wants with all its might to crush the star into a tiny ball.
When iron is finally forged, the fuel gauge hits empty. Iron can’t be fused to make heavier elements, which means no more heat is released in the core to keep gravity at bay. The star suddenly implodes — collapses in upon itself — and shortly after explodes as the rebounding shock wave tears it apart in the most amazing display of cosmic fireworks ever. Hello supernova!
Given their distances, it’s possible either or both Deneb and Antares have already exploded. Here on Earth, we wouldn’t know until their light arrives to announce the news.
Or it’s equally possible that both still have enough fuel to sustain a normal life for maybe a million more years. But either way, it’s almost certain that both will thrill skywatchers now or in the far future lucky enough to be around when the show begins.