Stella Nova, or Nova for short, is the Latin translation for “new star”. In astrophysical terms, the term nova does not mean newly formed objects, but rather old objects, more precisely: a pair of a white dwarf and a normal star or a giant star whose brightness suddenly increases. A white dwarf is the ultra-compact remnant of a once sun-like star in which the mass is compressed. If such a star corpse is in close orbit with an ordinary star, matter from the latter, mainly hydrogen gas, can reach the white dwarf. Because the stars move, this matter has angular momentum and initially collects in an accretion disk surrounding the white dwarf. Friction between the molecules then causes the gas to slowly lose energy until it finally rains down on the dwarf star and forms an increasingly dense shell on its surface. If the pressure becomes too great, thermonuclear fusion ignites and blows the envelope into space. The binary star system suddenly shines up to a million times brighter: from far away it looks as if there is a new star, a nova, in the sky.
In some (perhaps even in all) novae, this process repeats itself in a more or less regular rhythm. This year we have the unique chance to witness such an explosion from the start. For the first time, astronomers dare to predict the outbreak of a recurring nova. The affected binary star system is called T Coronae Borealis, or T CrB for short. It is in the constellation Northern Crown (Latin: Corona Borealis), which we can see in the evening in April in the east below the Bear Guardian with its bright star Arcturus: The Northern Crown is a semicircle of faint stars, whose brightest member, the star Gemma, is the reached second size class.
When can we expect the next Nova?
T CrB has been seen erupting four times in human history: in 1217, 1787, 1866 and 1946. There are exactly 80 years between the last two eruptions. If this time interval continues, the next Nova would be expected in 2026. Since the 19th century, astronomers have kept an eye on the double star, which is normally only visible with telescopes, and have found indications that the next nova of T CrB could appear between February and August 2024: a few months before its eruption in 1946 The brightness of the star system has dropped significantly. The reason for this is not known, but since spring 2023, observers from the American Association for the Study of Variable Stars (AAVSO) have been tracking a very similar “pre-eruption dip” to that seen before 1946. So it could start at any time. When it happens, everything happens very quickly: the increase in brightness only lasts a few hours – and suddenly there is a “new” star that is at least as bright, just below Gemma to the right. The T CrB nova from 1946 remains the brightest seen since then, and another eruption should be just as bright. After a few days, a week at the most, the nightmare is over again. T CrB will only be visible with telescopes for probably the next 80 years.
An evening look towards the Northern Crown is recommended to all stargazers in the next few weeks and months! On the AAVSO website, aavso.org, one can view the hourly updated brightness of T CrB. To do this you simply have to enter “T CrB” in the search field and click “check recent observations”. Among the observations, those corresponding to the visual impression made with the “Vis” or “V” filter. As soon as the values reach single digits and fall to two, the spectacle has begun.
Meanwhile, the April sky is clearly set for spring: Leo, Virgo and Bear Keeper dominate the southern and eastern parts of the firmament. Only in the far west can we still find the tough remnants of the winter sky. The Gemini in particular stand almost upright when looking west – Castor and Pollux, their brightest stars, form the heads of the mythical couple. The planets have completely said goodbye: Jupiter can still be seen at dusk, but it is already missing from the map calculated for 11 p.m. in the middle of the month. Anyone who sees Jupiter as a strikingly bright star low in the western sky at the end of twilight in the first three weeks of April should take a pair of binoculars: Comet 12P/Pons-Brooks is passing just to the right below the planet. This is prone to violent bursts of brightness, triggered by explosively sublimating ice beneath its porous surface. With a bit of luck you can see the comet as a foggy, round cloud in binoculars. On the evening of April 10th, around 9 p.m., Jupiter, 12P/Pons-Brooks and the very narrow crescent moon form a striking triangle. 12P/Pons-Brooks is expected to become even brighter by April 21st – but as we know, we never know exactly how bright with comets.
Sunrise and sunset
April 1st, sunrise 7:01 a.m., sunset 7:58 p.m.;
April 30th, sunrise 6:03 a.m., sunset 8:43 p.m.
Full moon and new moon
April 2, 5:15 a.m.: Last quarter;
April 8th, 8:21 p.m.: New moon;
April 15, 9:13 p.m.: First quarter;
April 24th, 1:49 a.m.: Full moon.
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