Ever since I started being interested in variables, these peculiar infant stars have fascinated me; here we are in the very cradle of creation, as a new star bursts into life out of its mother nebula, overseer of its birth-throes for millions of years. As with our own births, the process is gentle in thought but not in practice, and as the new-born star flares into life with many an unsure tremble, it blasts the parent gas and dust away into space, the so-called T Tauri wind. Billions of years ago, this is just what was happening to our own Sun before it decided to settle down into some sort of respectability in the galactic suburbs!
These stars used to be divided into three broad varieties, named after the prototypes and based on the appearance of the light-curve; the RW Aurigae stars, typified by continuous, unpredictable, and sometimes rapid changes in light; the T Orionis type, where the usual phase is maximum but where rapid and unpredictable falls and flickering take place; and last but not least those stars typified by T Tauri that show gentler, more sinusoidal changes in light. All of these stars are very young, and the T Tau stars especially are associated with embryonic dust clouds and discs characteristic of planet formation. This tripartite system of classification has since been shown to be too simple, however. Visually, some nebular variables can be seen to be rather fuzzy in appearance, such as V380 Orionis, not far from the Orion Nebula, itself the greatest concentration of Nebular variables in the sky.
In terms of observing nebular variables, like the cataclysmic stars, you need to estimate them on every possible occasion, notably the RW Aur objects, which are subject to very rapid light-changes. Currently, the AAVSO observes several of these stars, and selected light-curves appear on these pages. Now for a selection of my favourites!
This is one of the more active of the RW Aurigae stars as well as being among the brightest; its usual magnitude is around 9 to 9.6 so even a decent pair of binoculars may show it under good conditions. On one occasion I saw this star visibly change magnitude within the space of a minute - though as far as I know this was not confirmed by anybody else. Definitely a star to keep an unbiased eye on. It lies in the same low-power field as RR Tauri, a fainter but just-as-active star with a wider range in magnitude, from 10 to 14, as well as a less-active object, AD Tau. As you can see below, its variations can be extreme, rapid, and unpredictable. Quite easy to find near zeta Tauri, it lies inside a triangle of 5th-magnitude stars.
Another RW Aur star, this one is in the North of Orion in a field containing some other nebular stars, including the T Tau stars GW, HI and HK Orionis, the latter of which is decidedly fuzzy-looking. Currently not under study by amateur organisations, a chart for this star is shown here. Its official range is from magnitude 10 to 13, though it tends to hover around 11.5 as a rule in my limited experience of it so far.
This is a well-known T Tauri star, associated with a cometary-shaped nebula, as these objects often are. Most of the time it appears of about magnitude 10½ though its official range is from 8.6 to 11, and some particularly interesting changes are shown in the lightcurve presented here, which extends over about 6 months. A similar star, BP Tau, is close by. This one is normally about a magnitude fainter. Both stars show the gradual changes typical of T Tau stars, though RY is the more active; but BP has better comparisons.
Another underobserved star, this lies in a rich field very near the border with Eridanus inside a prominent triangle of 8.2, 8.8 and 10.3. It is the prototype of a particularly interesting subgroup of nebular stars, the UX Orionis variables (UXORs for short) which exhibit quasi-periodic fades from maximum. This object has a range of 8.7 - 12.8. You can obtain a chart from the AAVSO to follow its variations, which are of course unpredictable. In February 2000 the AAVSO ran a special project on this star. It is believed that the eclipse-like fades of the UX Orionis stars are caused by large clumpings of matter which are in the process of forming planets.
A star so new it doesn't even have a proper name yet! This is probably the most interesting variable I have ever observed. It appears to be a star of the UX Orionis type, so tends to spend a fair amount of time at maximum, which for this star is about magnitude 13. This is not a star for the fearful, both because of its faintness and the fact that it lies very close to a much brighter star of magnitude 8. About once a month, however, it plummets by sometimes as much as 3 magnitudes. On one occasion I caught it at magnitude 15.7 - and a friend observing from the USA later that same day estimated it at 14.2, so that it had risen by one and a half magnitudes in just a few hours! The minima tend to be sharp, which could suggest that if there are planets forming around it the dust clouds must be quite large. Maybe it is at a very early stage in the planet-forming process. You can delve deeper into the fascinating world of baby stars in the Young Stellar Objects part of the site. Here is a section from the lightcurve of MIS V1147 showing the deep minimum I observed. Note the sharp minima.
This is an interesting-looking T Tauri variable, probably of the subclass called the FU Orionis stars (FUORs), which are normally faint stars that undergo occasional outbursts as giant clouds of planet-forming material fall back onto them. It is in a diffuse nebula which gives the star a slightly fuzzy appearance. Once again this is an object not followed by regular amateur organisations as yet, so I have included a link to its chart for you. A variable that is very easily found between two 6th magnitude stars.
An unusual system, in that it is thought to be composed of two hot white stars, both of which are nebular-type variables. Add to that the fact that they also eclipse each other, and you have a really weird system! To make things even better, it is not terribly faint, varying between magnitudes 9.2 and 10.6, so may even be accessible with large binoculars. Furthermore, it is easy to find, making a large isosceles triangle with two stars both of magnitude 6.6. Naturally, I have included a chart for it, though I have to say that I have not followed it properly yet.
Last but not least, a T Tauri star in a region well-supplied with nebular variables; RW, HH and AB Aurigae lie not far off, all of them fairly bright objects, especially AB, which is entirely a binocular variable, as well as being a wide double whose companion is SU Aur, another nebular star. UY Aur is, like several other T Tauri stars, distinctly fuzzy. Its range is from 11.6 to 14.3 (pg) but I have tended to catch it at around magnitude 12.7 for the short time I have been observing it.