T Tauri and Hind's Nebula

John Russell Hind was educated as a civil engineer, but his intense interest in astronomy led him, in 1840, at the age of 17, away from engineering to a modest position at the Royal Observatory in Greenwich. A few years later he was appointed observer in the private observatory of Mr. George Bishop, a wealthy London business man. An exceedingly energetic observer, Hind is credited with the visual discovery of ten minor planets, three comets, and a number of new variable stars. But perhaps the discovery for which astronomers remember him best was made, rather casually on the night of October 11, 1852, while he was apparently searching for minor planets with Mr.Bishops 7-inch refractor. The next day he wrote to the Astronomische Nachrichten:
"Last night ... I noticed a very small nebulous-looking object in [he gave the position of a point in Taurus, just north of the Hyades]: it was south-preceding a star of 10th magnitude, which to my surprise, has escaped insertion on the map for 4h R.A. recently published, possibly it [the star] may be variable. The sky at that time was remarkably clear but the object appeared very faint ... I suppose it will prove a new nebula, none of our Catalogues having anything in the above position. Its diameter did not exceed 30"."

We now know the star as the variable T Tauri of which more will be said presently. In the next few years, Hind's nebula was observed by several astronomers, but in 1861, H. d'Arrest at Copenhagen found that it had disappeared, although he had seen it easily on a number of occasions in 1855 and 1856. Faint traces of Hind's nebula (which we now identify as number 1555 in the New General Catalogue of nebulae and clusters) were still discernible with the largest telescopes from 1861 to 1864, but by 1868 it had vanished completely.

But in 1868, Otto Struve found that nebulosity was present around a faint star of magnitude 14 that lies about 3' west of T Tauri. D'Arrest, who confirmed Struve's discovery, was certain that it had not been there before. Struve's nebula was visible until 1877, but it has not been seen since, and there is no sign of nebulosity at that position on modern photographs. Struve's nebula appears in the catalogues as N.G.C. 1554.

Apparently no more attention was paid to the region of T Tauri until 1890, when E.E. Barnard, together with S.W. Burnham, rediscovered Hind's nebula with the Lick 36-inch refractor. Burnham described it as "an excessively faint, round nebula about 3/4' from [T Tauri] ... it was ... apparently not connected with the variable, and was of nearly the last degree of faintness for the light-power of the large instrument. It is, perhaps, too faint for any other telescope:"-indeed a far cry from 1852, when Hind descovered it with a 7-inch. Barnard found that it had become a little brighter in early 1895, but by the fall of that year, it had utterly vanished in the 36-inch. In late 1897, Burnham and Barnard now using the Yerkes 40-inch suspected the presence of nebulosity at the very limit of vision, but it was an exceedingly difficult and uncertain observation even for such hawk-eyed observers.

In 1890, photography had been applied unsuccessfully to the problem by Isaac Roberts. In 1899, however J.E.Keeler with the Crossley 36-inch reflector at Lick found faint traces which corresponded as well as one might expect with the descriptions and sketches of NGC 1555 by Burnham and Barnard. Later photographs at Lick by H.D.Curtis and by F.G.Pease with the Mt.Wilson 60-inch reflector, in the period 1911-16, revealed more details. Pease also found that much of the dark nebula in which T Tauri and Hind's variable nebula lay was feebly luminous.

During the first period (from 1899 to about 1920) that photography had been used in the study of NGC 1555, and thus removed the large uncertainties unavoidably accompanying visual observation, unmistakable changes were observed in the appearance of Hind's nebula. Nevertheless, the nebula was always a very faint and difficult object, suitable for study only with the large reflectors. About 1920, one area of the nebula immediately to the west of T Tauri that on earlier photographs had been variable in intensity but always faint, began to brighten. By 1935, W.Baade and E.P.Hubble reported that it could be seen visually at the 100-inch telescope. C.O.Lampland, observing with the 42-inch reflector at Lowell Observatory, announced in 1936 that his observations suggested that the brightness of the nebula was on the increase. A published Mount Wilson photograph, taken by Baade with the 100-inch telescope in 1940, shows a spectacualr object composed of bright overlapping arcs, concave toward T Tauri. Photographs obtained with the Crossley reflector at Mount Hamilton since 1951 show approximately the same thing (Fig. 1). Hind's nebula is once again, a hundred years after its discovery, an easy visual object: it can now be seen without any difficulty with the Crossley reflector.

However, it is not simply a bright edition of the nebula shown on the early photographs. Then, the brightest area lay about 40 seconds of arc southwest of T Tauri, with much fainter nebulosity immediately to the west,and still fainter streamers farther away. The bright partof the nebula is now a long arc of nebulosity stretching from the southwest around to the northwest. The descriptions of Hind's nebula when it was so bright in 1852-56 show that, visually at least, the nebulosity lay southwest of T Tauri, in approximate agreement with the 1899 photographs. So although there can be no doubt that we are witnessing the rekindling of Hind's nebula, it seems to have reappeared in a somewhat different shape than it had when bright a century ago.

The association with T Tauri is so striking that one can hardly doubt that the variable star is somehow responsible for the illumination of the nebula. That the light of NGC 1555 is, principally if not entirely, reflected light of T Tauri is demonstrated by spectrograms of the nebulosity taken at the Lick Observatory in 1949 and 1951. These photographs show that the star and the nebula have spectra that are identical, as far as one can tell from spectrograms of necessarily low dispersion. This is quite in order, since the bright-line spectra of nebulae such as the Orion Nebula require for their production exciting stars of very high temperature. The nebulae associated with stars cooler than about 20,000C shine by reflected or scattered light alone. So, since T Tauri has the spectrum of a dwarf star somewhat cooler than the sun, one would expect Hind's nebula to have a reflection spectrum. We shall shortly be in for a surprise, however.

Let us go back to 1890 for a moment. Burnham, while examining the region with the Lick 36-inch refractor, discovered that T Tauri itself was imbedded in a very small, but bright elliptical nebulosity, elongated toward the southeast and extending outward only a few seconds of arc from the star. Burnham at first thought that this minute nebulosity was Hind's, but a few nights later Barnard saw and pointed out to Burnham the exceedingly faint patch of nebulosity about 45 seconds to the southward oof the star (already described), which was actually the remenant of NGC 1555. This inner nebulosity around T Tauri seems to be variable also! Barnard turned back to the position with the 36-inch 4 years after Burnham's discovery and found that, except for a "feeble indefinite nebulous glow" about the star, the bright elliptical nebula had vanished. With the 40-inch in 1897, Barnard saw a nebulous patch southeast of the star but only 1 or 2 seconds of arc from it that was probably part of Burnham's nebula, whichc had its greatest extension in that direction.

Photography brings both blessings and difficulties to the study of Burnham's nebulosity. While a long-exposure photograph can reveal faintly illuminated surfaces quite invisible to the eye, such an exposure also produces large overexposed discs for the brighter stars. The Crossley photographs taken by Keeler in 1899 show Burnham's nebulosity as a short fuzzy extension about 4 seconds of arc long of the overexposed T Tauri. It is probably present on the 1914-15 Crossley plates as well, but such a difficult object is properly in the domain of the largest reflectors, shich have both speed and large scale. By 1920 the 100-inch reflector at Mt. Wilson had become available, and with it Hubble found that there were at least three flares jutting out from the image of T Tauri. The brightest extends to the southwest. That to the southeast is probably part of Burnham's nebula, while the flare to the north may be also.

We have seen that Hind's nebula, at its closest 30 to 40 seconds from the star, shines by reflected light from T Tauri. There would seem no reason to think that the envelope of nebulosity close about the star would be any different. But in order to settle the question, spectroscopic observations are called for. This is a considerably more difficult problem than to obtain the spectrum of Hind's nebula, for which one needs only a fast spectograph and the patience to take an exposure of several hours in length. The nebulosity emmediately around T Tauri is not so faint, but it is nearly lost in the glare of the variable, so that sonsiderable care must be taken to keep stray light of the bright star from obliterating the spectrum of the nebula. Spectrograms of the inner nebulosity were obtained on two good nights in the winter of 1948-49 with the 82-inch reflector at the McDonald Observatory in Texas. These photographs show that the spectrum of the nebula was composed of bright lines due to oxygen, sulfur, and hydrogen, which one ordinarily finds only in nebulae excited by very hot blue stars. Yet here they were in a nebula associated with T Tauri, a cool yellow dwarf. No entirely satisfactory explanation of this anomaly has yet been given. It does show that the inner and outer nebulosities at T Tauri are significantly different in physical conditions, for reasons we do not now understand.

We have been speaking here of distances measured in angular units: seconds and minutes of arc. What do they mean in linear units? Since the distance of T Tauri is about 450 or 500 light-years, an angle of one second of arc corresponds to about 150 astronomical units (an astronomical unit is the distance from earth to sun, or 93,000,000 miles). So, if the close nebulosity runs to 10 seconds from T Tauri, it means that it must extend to at least 1500 astronomical units (or almost 40 times the radius of Pluto's orbit) from T Tauri. Of course, it is unlikely that the extension of the nebula is perpendicular to our line of sight; if it is not, the corresponding linear distance may be much more than 1500 astronomical units. As its closest, Hind's nebula is over three times as far away from the star(again assuming that the projection factors are the same) as the outer edges of the inner nebulosity.

We have avoided to the last the central question: what makes these nebulae vary in light? It seems quite unlikely that the variations are due to the nebulosity having been swept away and then reformed. because the velocities required are improbably large. We cannot blame the light-changes of the nebula upon the irregular light-variations that have been observed in the star, for there seems to be little correlation between the two. For example, T Tauri has varied its light to only a minor extent since 1916, but during that time Hind's nebula has greatly increased in brightness. Experience with other similar nebulae associated with variable stars suggests that these changes are largely ones of variable illumination. it seems likely that the clouds, which were so brightly lit up by T Tauri, were not dissolved about 1861, but rather they disappeared when the shadow of something nearer the star swept across them. We are probably witnessing no more than the play of light and shadow on a relatively fixed curtain of dust clouds. It is tempting to think that the shadows are cast by moving clouds or condensations in the close nebulosity around the star, which seems a natural place for such an activity to take place. The changes in brightness of Hind's nebula may therefore be due to variations in T Tauri as seen from the nebula, but those variations are probably quite different from the fluctuations of the star that are visible from our direction.