Object 25: DENIS J104814.7-395606
Podcast release date: 13 July 2020
Right ascension: 10:48:14.6
Corresponding Earth location: The Tasman Sea approximately halfway between Australia and New Zealand
DENIS J104814.7-395606 (which, to save my sanity, I am going to call DENIS 1048 for short) was only discovered in 2001. It was found in a database of sources created by the DEep Near-Infrared Survey (DENIS) . The star was named after the survey; the numbers in the star's name are the coordinates of the star, although they don't quite match the coordinates that I got from the Simbad Astronomical Database .
What is really weird about this discovery is that the star is extremely close to the Solar System in astronomical terms. The closest star system to Earth is the Alpha Centauri system, which is at a distance of 4.36 light years (1.34 pc) . DENIS 1048 is at a distance of 13.21 light years (4.05 pc) [4,5]. I can't find a reliable list of the closest stars to the Solar System (although I can find a few unreliable lists), but based on what I have found, DENIS 1048 is definitely within the top 100 closest stars to the Solar System and probably among the top 50 closest.
A lot of people would expect that it would be very difficult to miss seeing a nearby star, as people would expect nearby stars to appear very bright as seen from Earth. This is partially true; some of the closest star systems, including Alpha Centauri, Sirius, and Procyon, are also among the brightest stars in Earth's nighttime sky. The reason why DENIS 1048 went undetected for so long is because it's very small and it radiates very little electromagnetic radiation in comparison to these other stars.
DENIS 1048 is among a class of stars called ultracool dwarf stars. If you create a plot of the properties of stars where you have the surface temperature on the x-axis and the total amount of energy radiated per unit time (also referred to as the luminosity) on the y-axis, stars that fuse hydrogen into helium in their cores will fall on a line that runs from the very bright / very hot end of the graph to the very faint / very cold part of the graph. Astronomers call this line the main sequence. The stars on this sequence that are brighter and hotter are also bluer in color and more massive, while the fainter and cooler stars are also redder and less massive. The Sun lies in the middle of the main sequence in these types of plots. It's surface temperature is about 5500 C. DENIS 1048 lies at the very faint and very cold end. It has a surface temperature of somewhere around 2400 C, it looks very red, and it is estimated to be somewhere around 0.08 times the mass of the Sun .
In fact, it's not even clear if DENIS 1048 is massive enough to trigger the fusion of hydrogen into helium in its core . If no fusion is taking place, this would make DENIS 1048 a brown dwarf. Technically, a brown dwarf is a type of object with a mass somewhere between the mass of a very small star and a very large planet; it is too small to be powered by fusion and too large to be considered an ordinary gas giant like Jupiter. Beyond that description, it's hard to give a precise definition of what a brown dwarf is. At the low mass end, it's not clear exactly when to say that an object is no longer a very small brown dwarf but instead a very large planet, like, for example, a very large version of Jupiter. At the high mass end, it's not clear as to how small a star can be before fusion shuts off in its core and it gets declared a brown dwarf. This applies to DENIS 1048, where it seems too warm and too massive to definitely be a brown dwarf with no fusion in its core but too cool and too small to definitely be a star that is actively fusing hydrogen into helium. To allow astronomers to write science papers while avoiding endless semantics arguments, astronomers refer to DENIS 1048 and similar stars as ultracool dwarf stars.
DENIS 1048 is a nearby example of an entire population of ultracool dwarf stars that may not produce much light individually or even together but may constitute a significant fraction of the total mass of stars in our galaxy. Hence, knowing that these ultracool dwarf stars are out there is important, and to properly understand the stellar populations within our galaxy, astronomers need to find more stars like DENIS 1048. However, to understand more distant ultracool dwarf stars, astronomers are going to look first at DENIS 1048 as one of the closest examples.
 Delfosse, X. et al., New neighbours: II. An M9 dwarf at d ~ 4 pc, DENIS-P J104814.7-395606.1, 2001, Astronomy & Astrophysics, 366, L13
 SIMBAD Astronomical Database, 2020
 Kervella, P. et al., Close stellar conjunctions of alpha Centauri A and B until 2050 . An mK = 7.8 star may enter the Einstein ring of alpha Cen A in 2028, 2016, Astronomy & Astrophysics, 594, A107
 Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1
 Gaia Collaboration et al., Gaia Data Release 2. Summary of the contents and survey properties, 2018, Astronomy & Astrophysics, 616, A1
 Neuhauser, R. et al., Deep infrared imaging and spectroscopy of the nearby late M-dwarf DENIS-P J104814-395606, 2002, Astronomische Nachrichten, 323, 447