"The extreme temporary light emissions of a nova are often visible to the naked eye from Earth. Even with several spectacular visible phenomena, such as this night sky, the nova remains recognizable to the trained eye. Due to their short-term glow and later dimming, it seemed to earlier cultures as if there were celestial bodies that could appear and disappear out of nowhere. In ancient China, astronomers called these 'Guest Stars' More on this in the next post"

2021 August 16

Perseid Meteor, Red Sprites, and Nova RS Ophiuchus
* Image Credit & Copyright: Daniel Korona

Explanation:
This was an unusual sky. It wasn't unusual because of the central band the Milky Way Galaxy, visible along the image left. Most dark skies show part of the Milky Way. It wasn't unusual because of the bright meteor visible on the upper right. Many images taken during last week's Perseid Meteor Shower show meteors, although this Perseid was particularly bright. This sky wasn't unusual because of the red sprites, visible on the lower right. Although this type of lightning has only been noted in the past few decades, images of sprites are becoming more common. This sky wasn't unusual because of the nova, visible just above the image center. Novas bright enough to be seen with the unaided eye occur every few years, with pictured Nova RS Ophiuchus discovered about a week ago. What was most unusual, though, was to capture all these things together, in a single night, on a single sky. The unusual sky occurred above Zacatecas, Mexico.

https://apod.nasa.gov/apod/ap210816.html

What Will It Look Like When Betelgeuse Goes Supernova?

640 light years away, there is a colossal star that is on the verge of exploding in a magnificent supernova. But what will it look like from Earth?

Video Credit:
V101 SPACE
https://www.youtube.com/@V101SPACE

To upload this video, I converted it and compressed it to less than a tenth of the original size under linux with the free software ffmpeg and the corresponding command:
'ffmpeg -i video_in.webm -vcodec libx265 -crf 40 video_out.mp4'
Maybe you would like to post a corresponding video on a scientifically related topic, but it is perhaps too big? Then try ffmpeg. Often the compression factor 'crf -28' is enough, but in this particular case I had to go up to 'crf -40', because the original is a 4K UHD video and has a file size of 377MB. The compressed version here has a file size of only 28.4MB .
Of course, the quality of the video decreases, but it is enough to illustrate the topic.

Want to see the video in all its glory? Then visit the channel https://www.youtube.com/@V101SPACE and let Rob explain exciting space-related topics to you

2020 May 11

Behind Betelgeuse
* Image Credit & Copyright: Adam Block, Steward Observatory, University of Arizona
https://www.adamblockphotos.com/
https://astro.arizona.edu/

Explanation:
What's behind Betelgeuse? One of the brighter and more unusual stars in the sky, the red supergiant star Betelgeuse can be found in the direction of famous constellation Orion. Betelgeuse, however, is actually well in front of many of the constellation's other bright stars, and also in front of the greater Orion Molecular Cloud Complex. Numerically, light takes about 700 years to reach us from Betelgeuse, but about 1,300 years to reach us from the Orion Nebula and its surrounding dust and gas. All but the largest telescopes see Betelgeuse as only a point of light, but a point so bright that the inherent blurriness created by the telescope and Earth's atmosphere make it seem extended. In the featured long-exposure image, thousands of stars in our Milky Way Galaxy can be seen in the background behind Betelgeuse, as well as dark dust from the Orion Molecular Cloud, and some red-glowing emission from hydrogen gas on the outskirts of the more distant Lambda Orionis Ring.

Betelgeuse has recovered from appearing unusually dim over the past six months (2020), but is still expected to explode in a spectacular supernova sometime in the next (about) 100,000 years.

https://apod.nasa.gov/apod/ap200511.html

KT Eridani
From Wikipedia, the free encyclopedia

KT Eridani (Nova Eridani 2009) was a bright nova in the constellation Eridanus that produced an outburst in 2009. It was the first classical nova ever detected in that constellation. The nova was discovered at 12:52 UT on 25 November 2009 by K. Itagaki at Yamagata, Japan with a 21 cm patrol telescope. At the time of its discovery, it was a magnitude 8.1 object. The discovery occurred after the nova's peak brightness, but the All Sky Automated Survey system had detected the nova on three earlier occasions, allowing a more complete light curve to be produced. The peak magnitude, 5.4, was seen at 15:10 UT on 14 November 2009, at which time it would have been visible to the naked eye.

A very high temporal resolution light curve, beginning on 13 November 2009, was obtained from images taken by the Solar Mass Ejection Imager on the Coriolis satellite. These observations show that the peak brightness, magnitude 5.42±0.02, occurred at November 14.67±0.04 UT. The satellite was able to detect the nova until November 27.23±0.04, by which time it had dropped to magnitude 8.3±0.1. It is classified as a very fast nova, meaning it dimmed rapidly after peak brightness.

On 28 December 2009 (44 days after peak brightness) it was detected as a 0.21 milliJansky source at 5 GHz by the Very Large Array. In the following weeks its radio brightness increased and it was detected in additional radio bands. On the other end of the electromagnetic spectrum, the Swift satellite was used to look for X-ray emission from KT Eridani starting on 27 November 2009, and the satellite detected it on 24 December 2009.

GK Persei
From Wikipedia, the free encyclopedia

GK Persei (also Nova Persei 1901) was a bright nova first observed on Earth in 1901. It was discovered by Thomas David Anderson, an Edinburgh clergyman, at 02:40 UT on 22 February 1901 when it was at magnitude 2.7. It reached a maximum magnitude of 0.2, the brightest nova of modern times until Nova Aquilae 1918. After fading into obscurity at about magnitude 12 to 13 during the early 20th century, GK Persei began displaying infrequent outbursts of 2 to 3 magnitudes (about 7 to 15 times quiescent brightness). Since about 1980, these outbursts have become quite regular, typically lasting about two months and occurring about every three years. Thus, GK Persei seems to have changed from a classical nova like Nova Aquilae 1918 to something resembling a typical dwarf nova-type cataclysmic variable star.

Surrounding GK Persei is the Firework Nebula, a nova remnant first detected in 1902 consisting of an expanding cloud of gas and dust bubbles moving up to 1200 km/s.

Please see ALT_TEXTS for more Details

"Mini Supernova" Explosion Could Have Big Impact - NASA

In Hollywood blockbusters, explosions are often among the stars of the show. In space, explosions of actual stars are a focus for scientists who hope to better understand their births, lives, and deaths and how they interact with their surroundings.
[...]
In 1901 when it suddenly appeared as one of the brightest stars in the sky for a few days, before gradually fading away in brightness.
Today, astronomers cite GK Persei as an example of a “classical nova,” an outburst produced by a thermonuclear explosion on the surface of a white dwarf star, the dense remnant of a Sun-like star.

A nova can occur if the strong gravity of a white dwarf pulls material from its orbiting companion star. If enough material, mostly in the form of hydrogen gas, accumulates on the surface of the white dwarf, nuclear fusion reactions can occur and intensify, culminating into a cosmic-sized hydrogen bomb blast. The outer layers of the white dwarf are blown away, producing a nova outburst that can be observed for a period of months to years as the material expands into space.

Classical novas can be considered to be “miniature” versions of supernova explosions. Supernovas signal the destruction of an entire star and can be so bright that they outshine the whole galaxy where they are found. Supernovas are extremely important for cosmic ecology because they inject huge amounts of energy into the interstellar gas, and are responsible for dispersing elements such as iron, calcium and oxygen into space where they may be incorporated into future generations of stars and planets.

Although the remnants of supernovas are much more massive and energetic than classical novas, some of the fundamental physics is the same. [...]
https://www.nasa.gov/image-article/mini-supernova-explosion-could-have-big-impact/

2024 April 30

GK Per: Nova and Planetary Nebula
* Image Credit & Copyright: Deep Sky Collective
https://deepskycollective.com/home

Explanation:
The star system GK Per is known to be associated with only two of the three nebulas pictured. At 1500 light years distant, Nova Persei 1901 (GK Persei) was the second closest nova yet recorded. At the very center is a white dwarf star, the surviving core of a former Sun-like star. It is surrounded by the circular Firework nebula, gas that was ejected by a thermonuclear explosion on the white dwarf's surface -- a nova -- as recorded in 1901. The red glowing gas surrounding the Firework nebula is the atmosphere that used to surround the central star. This gas was expelled before the nova and appears as a diffuse planetary nebula. The faint gray gas running across is interstellar cirrus that seems to be just passing through coincidently. In 1901, GK Per's nova became brighter than Betelgeuse. Similarly, star system T CrB is expected to erupt in a nova later this year, but we don't know exactly when nor how bright it will become.

https://apod.nasa.gov/apod/ap240430.html

"EXTREMELY RARE"--TWO NAKED-EYE NOVA (UPDATED): A person could go a lifetime without seeing a star explode with their own eyes. Right now, people in the southern hemisphere can see two stars exploding at the same time. The first (V462 Lupi) appeared on June 12th in the constellation Lupus, and the second (V572 Velorum) on June 25th in Vela. They're both brighter than 6th magnitude, the threshold for naked-eye visibility.

"This is without question an extremely rare event, if not an historical one," says astronomer Stephen O’Meara, who has been scouring historical records for the last time this happened. "I have yet to find an occurrence of two simultaneous nova appearing at the same time."

"I thought I had found a pair in 1936 (V630 Sgr and V368 Aql)," he says. "But I looked at their light curves, and it turns out they were not at maximum brightness at the same time."

These appear to be classical novas. First documented by Chinese astronomers some 2000 years ago, the explosions occur in binary star systems. White dwarf stars steal gas from a bloated partner until the stolen fuel ignites in a sudden thermonuclear blast. It's less dramatic than a supernova, but still an awesome blast.

Nova V462 Lupi is currently near magnitude +5.9, while V572 Velorum, fading from a peak near +4.8, is the brighter of the two. Southern astronomers, this may not happen again for a loooong time. Submit your photos here.

Update: "I have found another event in AAVSO archives about seventeen years ago," says O’Meara. "On March 22, 2018, Nova Circinis 2018 reached a peak brightness of magnitude 5.8 , and on that same day, Nova V906 Carinae peaked at around magnitude 5.9. So these two novae shared a similar naked-eye magnitude albeit briefly. So these are rare events."

https://spaceweather.com/archive.php?view=1&day=30&month=06&year=2025
https://spaceweathergallery2.com/indiv_upload.php?upload_id=223885

What’s a Nova? Inside the Chaos of Erupting and Exploding Stars
Chelsea Gohd, NASA Universe Web Team

In short, a nova is an outburst in a binary system, or system with two stars that closely orbit one another.

In all known novae, one of the stars in the system is a white dwarf, which is the leftover core of a star like our Sun after it runs out of fuel. Its companion will typically be either a main sequence star, a classification that includes 90% of known stars, or a large star like a red giant approaching the end of its life.

In most systems that create novae, the white dwarf and its companion star orbit so closely that the white dwarf’s gravity pulls material from the companion. This material swirls around the white dwarf and forms an accretion disk. Throughout this process, the white dwarf's gravity will pull the material toward itself, depositing it onto its own surface, which while fluid in consistency is extremely hot and dense.
The material pulled from the atmosphere of the companion star is primarily hydrogen. Over time, that material builds up on the surface of the white dwarf. As hydrogen accumulates, it is heated by the white dwarf, and that heat builds until it eventually triggers a fusion reaction. In other words, the gas is explosively ejected from the surface of the white dwarf.
Novae are, on average, around 200,000 times as bright as the Sun, while the brightest novae can be 10 times brighter than that. Novae can even be bright enough to be seen with the unaided eye from Earth, even if the star system wasn’t visible prior to the outburst. Novae can last for days, weeks, or even months.
There is variety among novae, as they can take place with different types of companion stars and can last for different amounts of time and with varying levels of brightness. However, there are two main types: classical and recurrent[...]
https://science.nasa.gov/universe/whats-a-nova-inside-the-chaos-of-erupting-and-exploding-stars/