Here is possibly the only joke in all of astronomy. Two spiral galaxies walk into a pub. The first one goes up to the bar and asks for two drinks, one for him and one for his friend. The barman looks suspiciously at the second spiral galaxy and says “I’m sorry but I can’t serve your friend, he’ll have to go.” The first spiral galaxy complains, “Why can’t you serve him?” The barman replies, “He’s barred.” Bazinga!*
Get it? He’s a barred spiral galaxy! Many spiral galaxies have bars of stars, gas and dust running across their centres. We have even discovered that our own Milky Way has a modest central bar. However the bar in NGC 1672, shown in the spectacular Hubble Space Telescope image above, is a much more prominent feature. Galaxies are islands of stars and dust in a Universe otherwise empty of ‘ordinary’ matter. A galaxy can contain between a million and a trillion stars. Galaxies like the Milky Way and NGC 1672 are classed as spirals (although of differing sub-type). The spiral arms are made of stars and gas pressed together in ‘density waves’ where their average speed is less than the galaxy’s rotational speed. Bars are believed to be temporary structures caused by a density wave radiating from the galaxy’s centre.
Apart from the bar, the image shows all the familiar galactic elements. There is a bright nucleus containing a supermassive black hole, concealed by the orange glow of the old Population II stars gathered in the core. Powerful radio emissions from its centre and bar reveal that the supermassive black hole is ravenously gobbling matter. As this material, dust, gas, and perhaps even stars, is swallowed vast quantities of radiation are emitted. This radiation could sterilise planets thoughout this galaxy. We can also see dark wispy dust lanes, scattered nebulae glowing in the distinctive red of hydrogen and clusters of bright blue new stars. One day these dust lanes will form stars and planets. Perhaps by then the supermassive black hole will be dormant again and life will arise anew in NGC1672.
NGC 1672, is about 75 000 light years (23 kpc) across, somewhat smaller than the Milky Way. It is 60 million light years (18 Mpc) from us in the Southern Hemisphere constellation Dorado (the Swordfish).
Anyone out there want to share your favorite astronomical joke?