Deep-sky objects are celestial objects that exist outside our solar system. Three major types of deep-sky objects are nebulae, star clusters, and galaxies.

We use the dynamics of a galaxy, set up initially at a constant proper distance from an observer, to derive and illustrate two counter-intuitive general relativistic results. Although the galaxy does gradually join the expansion of the universe (Hubble flow), it does not necessarily recede from us. In particular, in the currently favored cosmological model, which includes a cosmological constant, the galaxy recedes from the observer as it joins the Hubble flow, but in the previously favored cold dark matter model, the galaxy approaches, passes through the observer, and joins the Hubble flow on the opposite side of the sky. We show that this behavior is consistent with the general relativistic idea that space is expanding and is determined by the acceleration of the expansion of the universe-not a force or drag associated with the expansion itself. We also show that objects at a constant proper distance will have a nonzero redshift; receding galaxies can be blueshifted and approaching galaxies can be redshifted.

Both Centaurs and trans-Neptunian objects (TNOs) are minor bodies found in the outer Solar System. Centaurs are a transient population that moves between the orbits of Jupiter and Neptune, and they probably diffused out of the TNOs. TNOs move mainly beyond Neptune. Some of these objects display episodic cometary behaviour; a few percent of them are known to host binary companions. Here, we study the light-curves of two Centaurs -2060 Chiron (1977 UB) and 10199 Chariklo (1997 CU26)- and three TNOs -38628 Huya (2000 EB173), 28978 Ixion (2001 KX76), and 90482 Orcus (2004 DW)- and the colours of the Centaurs and Huya. Precise, ~1%, R-band absolute CCD photometry of these minor bodies acquired between 2006 and 2011 is presented; the new data are used to investigate the rotation rate of these objects. The colours of the Centaurs and Huya are determined using BVRI photometry. The point spread function of the five minor bodies is analysed, searching for signs of a coma or close companions. Astrometry is also discussed. A periodogram analysis of the light-curves of these objects gives the following rotational periods: 5.5+-0.4 h for Chiron, 7.0+-0.6 h for Chariklo, 4.45+-0.07 h for Huya, 12.4+-0.3 h for Ixion, and 11.9+-0.5 h for Orcus. The colour indices of Chiron are found to be B-V=0.53+-0.05, V-R=0.37+-0.08, and R-I=0.36+-0.15. The values computed for Chariklo are V-R=0.62+-0.07 and R-I=0.61+-0.07. For Huya, we find V-R=0.58+-0.09 and R-I=0.64+-0.20. We find very low levels of cometary activity (if any) and no sign of close or wide binary companions for these minor bodies.

We present a multi-wavelength study of Extremely Red Objects (EROs) employing deep RIzJHK photometry of a 8.5'x8.5' region to identify 68 EROs with R-K>5.3 and K10^23 W/Hz at z>1 or a SFR of >25Mo/yr. We detect radio emission from 21 EROs at >12.6uJy and resolve a third of these with our 1.6'' FWHM beam. The SEDs of most of these radio EROs are consistent with dust-reddened starbursts at z~1. At z~1 the radio luminosities of these EROs indicate far-infrared luminosities of L_FIR>10^12 Lo, meaning half are ultraluminous infrared galaxies. We conclude that >16+/-5% of EROs with K1. We also photometrically classify the EROs to investigate the mix of dusty/active and evolved/passive systems in the radio-undetected EROs. We suggest that at least 30%, and perhaps up to ~60%, of all EROs with R-K>5.3 and K1. The SFD in this optically faint (R>26) population is rho^* (0.1-100Mo)=0.11+/-0.03 Mo/yr/Mpc^3, comparable to that in H-alpha emitting galaxies at z~1, and greater than that in UV-selected samples at these epochs. This support the claim of a strong increase in obscured star formation at high redshifts. Using the observed counts of the radio-detected EROs we model the break in the K-band number counts of all EROs at K~19-20 and propose that the passive ERO class dominates the total population in a narrow range around K~20, with dusty EROs dominating at fainter magnitudes. [Abridged]

We report the observational analyses and theoretical interpretations of unusually red galaxies in the Subaru Deep Field (SDF). A careful analysis of the SDF data revealed a population with unusually red near-infrared (NIR) colors of J-K>~3-4, with a higher confidence than the previous SDF result. Their surface number density drastically increases at K>~22 and becomes roughly the same as that of dusty starburst galaxies detected by submillimeter observations in recent years. These colors are even redder than the known population of the extremely red objects (EROs) and are too red to be explained as passively evolving elliptical galaxies, which are the largest population of EROs. Hence, these hyper extremely red objects should be considered as a distinct population from EROs. We discuss several possible interpretations of these enigmatic objects, and we show that these red NIR colors, the K-band and submillimeter flux, and the surface number density are quantitatively best explained by primordial elliptical galaxies reddened by dust and still in the starburst phase of their formation at z~3. Partially based on the data corrected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

It is shown that Ton 256 (z = 0.131) and PKS 2251+11 (z = 0.323) are associated with galaxies of essentially the same redshift, thus implying that these objects are at cosmological distances. The nature of Ton 256 is questionable, and it is argued that it represents a transition between Seyfert galaxies and QSOs. There is no doubt that PKS 2251+11 is a bona fide QSO; it is bright (M = -24.7), blue (B - V = 0.20, U - B = -0.84), has a starlike image under high resolution, and is a strong radio source. A peculiar emission structure near PKS 2251+11 is also discussed.

We report the detection of water ice in the Centaur 2060 Chiron, based on near-infrared spectra (1.0 - 2.5 micron) taken with the 3.8-meter United Kingdom Infrared Telescope (UKIRT) and the 10-meter Keck Telescope. The appearance of this ice is correlated with the recent decline in Chiron's cometary activity: the decrease in the coma cross-section allows previously hidden solid-state surface features to be seen. We predict that water ice is ubiquitous among Centaurs and Kuiper Belt objects, but its surface coverage varies from object to object, and thus determines its detectability and the occurrence of cometary activity.

Radio positions of the three sources have been determined with the two 90-foot antennas working as an interferometer with an r.m.s. accuracy in both co-ordinates better than 10 seconds of arc. Direct photographs show that a starlike object exists within the error rectangle at each of the source positions. Exceedingly faint wisps of nebulosity are associated with the stars in 3C 48 and 3C 196. The observations are incomplete for 3C 286 in this regard. Photoelectric photometry of the stars shows each to have quite peculiar color indices, most closely resembling the colors of old novae or possibly white dwarfs, but we are not suggesting identification with these types of stars. Photometry of 3C 48 through 13 months shows the star to be variable by at least AV = 0W4. The radio flux appears to be constant. Optical spectra for 3C 48 show several very broad emission features, the most intense at X 3832 being unidentified. Spectra by Schmidt of 3C 196 and 3C 286 show other unusual features. The radio structure of the three radio stars is similar in that each has an unresolved core of <1" diameter. However, 3C 196 and 3C 286 show halos of 12" and 20", respectively, while no radio halo has been detected for 3C 48. it is shown that the radiant flux in the optical region can be computed from the radio-flux data and the theory of synchrotron radiation for 3C 48 and 3C 196, but not for 3C 286. This, together with other arguments, suggests that the optical as well as the radio flux could be due to the synchrotron mechanism, but the arguments are not conclusive. We have used the assumption of minimum total energy to compute the energy in relativistic particles and magnetic field required by the synchrotron mechanism to explain the observed emission. The magnetic field in each of the core components is near 0.1 gauss and depends mainly on the assumed angular size of the emitting region. The total energy in the core components is near 10 ergs. The rate of radiation is such that the energy in relativistic electrons must be replaced in a time scale of a few years if the value of the magnetic field determined in this way is correct. These calculations are based on a distance of 100pcs. The frequency of occurrence of radio stars is examined, and they are estimated to comprise from 5 to 10 per cent of sources in the 3C catalogue. The percentage is likely to be less for fainter sources. Rough limits have been estimated for the mean distances of these radio stars. A mean distance of approximately 100 pc is suggested if these objects are in the Galaxy. Evidence obtained since this paper was written suggests that 3C 48 has a large redshift of z = 0.3675 (Greenstein and Matthews 1963); thus these objects may be associated with a distant galaxy. The absolute magnitude of the starlike objects is M = -24.3, which is much brighter than any other known galaxy. As a radio source, 3C 48 is not very different from other identified sources. The emitted flux is the same as 3C 295 and Cygnus A, but the emitting volume is much less. The faint nebulosity does not resemble a galaxy, and it also is brighter than a normal galaxy. if caused by an explosion in the past and expanding at the velocity of light, its age would be > 1.8 X 10 years. The synchrotron lifetime calculated in the normal manner is much shorter than that inferred from the extent of the faint nebulosity. Thus either the magnetic field must be much lower than calculated, or high-energy electrons must be supplied continuously.

The universe is packed with exotic objects, including black holes, pulsars, gamma-ray bursts, magnetars, quasars, and more.