How big is the Bubble Nebula?

The Bubble Nebula is a round shell of glowing gas blown into space by a hot, massive star. It lies about 9,000 light-years from Earth in the constellation Cassiopeia. The bubble is about 7.5 light-years across, or about 70 trillion kilometers. A light-year is the distance light travels in one year, about 9.46 trillion kilometers.

Size

The bubble looks almost perfectly round in the sky, about 3 arcminutes across, where an arcminute is 1/60 of a degree. Its true width depends on its distance.

A 2019 study took the distance to its star from the Gaia spacecraft, about 2,700 parsecs. From that it found a diameter of 2.3 parsecs, give or take 0.2. A parsec is about 3.26 light-years, so the bubble is about 7.5 light-years across.

A later Gaia data release places the star a little farther away, about 2,800 parsecs or 9,200 light-years, which would make the bubble about 8 light-years across. Earlier descriptions from NASA and the European Space Agency gave smaller distances, between 7,000 and 8,000 light-years. They gave widths between 6 and 10 light-years.

Size comparisons

• The bubble is about 10% wider than the Cat's Eye Nebula, which is about 6.9 light-years across including its faint halo.
• It is about 1.8 times the distance from the Sun to Proxima Centauri, which is about 4.2 light-years.
• The star that blows the bubble is about 15 times as wide as the Sun, and the bubble is about 3 million times as wide as the star.
• If the Sun were shrunk to the size of a basketball, the bubble would be about 12,000 kilometers (7,600 miles) across, about as wide as Earth.

The star that blows the bubble

The star, called BD+60°2522, is young, very bright and massive, of the kind astronomers call O-type. A 2020 study estimated that it has about 27 times the mass of the Sun. Its surface gas is so hot that it streams away into space as a stellar wind. The same study measured the wind's speed at about 2,000 kilometers per second, or about 7 million kilometers per hour.

The wind sweeps up the cold gas in front of it, like a snowplow piling up snow, and the swept-up gas forms the bubble's outer edge. The star sits off-center. One explanation is that on one side the shell runs into denser cold gas. The star is about four million years old and will probably explode as a supernova, the explosion of a massive star, in 10–20 million years.

Discovery

William Herschel discovered the Bubble Nebula in November 1787. It is so large in the sky that early Hubble Space Telescope images showed only parts of it. In 2016, to mark 26 years in orbit, Hubble combined four images to show the whole bubble in one picture for the first time.

Sources

• Thermal emission from bow shocks I: 2D Hydrodynamic Models of the Bubble Nebula. Green et al., Astronomy & Astrophysics, 2019.
• The Bubble Nebula NGC 7635 – testing the wind-blown bubble theory. Toalá et al., Monthly Notices of the Royal Astronomical Society, 2020.
• Gaia EDR3 distance to BD+60°2522, source 2014149897293278848. Bailer-Jones et al., via CDS VizieR, 2021.
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3. Bailer-Jones et al., The Astronomical Journal, 2021.
• Hubble Sees a Star 'Inflating' a Giant Bubble. NASA, 2016.
• Hubble captures birthday bubble. ESA/Hubble, 2016.
• Caldwell 11. NASA Hubble Space Telescope, Hubble's Caldwell Catalog.
• The giant haloes of NGC 6543 and 6826. Middlemass, Clegg and Walsh, Monthly Notices of the Royal Astronomical Society, 1989.
• Gaia EDR3 distance to the central star of the Cat's Eye Nebula, source 1633325248915154176. Bailer-Jones et al., via CDS VizieR, 2021.
• The Nearest Neighbor Star. NASA Goddard Space Flight Center, Imagine the Universe, 2020.
• Measuring the Universe. International Astronomical Union.
• Sun Fact Sheet. NASA, 2024.
• Earth Fact Sheet. NASA, 2024.
• Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.