The Sombrero Galaxy

Degree of difficulty 3 (of 5)

Minimum aperture 50mm

Designation NGC 4594

Type Galaxy

Class Sa

Distance 44.7 Mly (2003) 27.9 Mly (2001) 31.9 Mly (2001)

Size 105,000 ly

Constellation Virgo

R.A. 12h 40.0min

Decl. –11° 37′

Magnitude 8.0

Surface brightness 20.5mag/arcsec2

Apparent diameter 8,7′×3,5′

Discoverer Méchain, 1781

History M 104 was discovered on the 11th of May 1781, by Pierre Méchain, who noted: “nebula which did not appear to me to contain any single star. It is of a faint light.” Messier knew about Méchain's discovery, since he added a hand-written note of it to his printed copy of his final catalog, and it is likely that he observed this object himself. However, this object came too late to make it into the original catalog. Finally, the well-known French popular astronomy writer Camille Flammarion found Messier's note in 1921 and awarded this galaxy the merits of a Messier object.

William Herschel was able to see much more of M 104 than Méchain. He described it as “A faint diffused oval light” and observed the dark lane bisecting the galaxy. His son John confirmed in 1833 “that there is a dark interval or stratum separating the nucleus and general mass of the nebula from the light above it. Surely no illusion.”

A few years later, Admiral Smyth speculated: “This must be another of those vast flat rings seen very obliquely, already spoken of, and is an elegant example of that celestial perspective.” Heinrich d'Arrest, however, saw just a “bright ray, small nucleus like a star of 10th magnitude.” It took the first deep photographic exposures in the early 20th century to reveal the full beauty of this galaxy. Curtis wrote: “A remarkable, slightly curved, clear-cut dark lane runs along the entire length to the south of the nucleus; probably the finest known example of this phenomenon.

Degree of difficulty 3 (of 5)

Minimum aperture 30mm

Designation NGC 581

Type Open cluster

Class II2m

Distance 7150 ly (K2005) 9790 ly (CMD, 1999) 5220 ly (CMD, 1998)

Size 17 ly

Constellation Cassiopeia

R.A. 1h 33.4min

Decl. +60° 40′

Magnitude 7.4

Surface brightness –

Apparent diameter 6′

Discoverer Méchain, 1781

History M 103 is the last entry of Messier's original list. It made it into a last-minute annex with M 101 and M 102, after its discovery by Pierre Méchain on the 27th of March 1781, when the catalog of originally 100 nebulous objects was already set up for printing. Méchain recognized it correctly as a “Cluster of stars between ε and δ of the leg of Cassiopeia.”

50 years later, Admiral Smyth described M 103 as a “fan-shaped group, diverging from a sharp star in the north following [northeast] quadrant. The cluster is brilliant from the splash of a score of its largest members, the four principle ones of which are from the 7th to the 9th magnitude; and under the largest, in the southeast, is a red star of the 8th magnitude.” A similar view, expressed in fewer words, had d'Arrest: “A pretty, reddish star stands out, of a rosé tint. An uneven cluster, consisting of stars of 9th, 10th, and 11th magnitude.” Leo Brenner observed M 103 with a 7-inch telescope and noticed its double star: “pretty with magnification 93×, triangular, with pretty double star magnitudes 6 and 10, 13″ to 14″ separation, furthermore a beautiful red star of 8th magnitude, otherwise stars of 9th to 10th magnitudes.”

Astrophysics With a likely distance of 7200 light-years, M 103 is about as far away as the famous double cluster h and χ in the Perseus arm of the Milky Way, and it is the farthest open cluster in the Messier catalog.

So far, we have focused our attention mainly on the properties of individual sunspots and starspots, and their associated magnetic fields. Now we turn to systematic variations in magnetic activity and in the incidence of spots. In this chapter we shall only consider observable manifestations of activity. The magnetic fields that emerge through the surface of a star are actually generated in its interior, by dynamo processes which will be discussed in Chapter 11.

We begin by describing the well-known sunspot cycle, with an average period of about 11 years, which was first recognized by Schwabe (as explained in Section 2.2). This cycle is apparent in the record of telescopic observations, though it was interrupted during the Maunder Minimum in the seventeenth century. Fortunately, the record can be extended back through hundreds, thousands and tens of thousands of years by using measured abundances of cosmogenic isotopes as proxy data. These data confirm that similar grand minima are a regular feature of solar activity, and we can explore their statistical properties.

Next, we turn to other Sun-like stars. As expected, they can exhibit activity cycles too, although these are most apparent in middle-aged slow rotators, like the Sun itself. Younger, more rapidly rotating stars are much more active but their behaviour is erratic and less obviously periodic, as can be seen in Figure 1.7.

10.1 Cyclic activity in the Sun

Figure 2.3 shows how the area covered by sunspots has varied over the past 130 years (since the daily Greenwich photoheliographic record was initiated).

Less than a few hundred thousand years after the Big Bang, the temperature was high enough that cosmic gas consisted of protons, free electrons and light nuclei. Once the Universe cooled to about 3000 K, the electrons and protons were moving sufficiently slowly that they combined to form hydrogen atoms. With scattering of photons much reduced, they were able to move in straight lines indefinitely, and may be seen redshifted into the microwave part of the spectrum as the 2.7K CMB. So began the era of recombination, or so-called “dark ages” when the IGM became mostly neutral. Within the current cold Dark Matter model for the hierarchical formation of structure, mini-halos of mass ∼106M⊙ (Couchman & Rees 1986) provided the gravitational seeds for the first stars at z ≈ 20–30, ending the “dark ages” through re-ionization of the IGM. A comprehensive review of the astrophysical role of dark matter is provided by Jungman, Kamionkowski, & Griest (1996).

Galaxies formed as baryonic gas cooled in the centers of dark matter structures, from which galaxy mass built up via mergers of halos and proto-galaxies (White & Rees 1978; Davis et al. 1985). Since most present-day galaxies are relatively old, it follows that they formed at z ≥2. The timescale over which galaxies assembled remains unclear, particularly the bulges and disks which are the main components of present-day galaxies.

The many results that we have described amply demonstrate the rapidly accelerating rate of progress in our knowledge of the properties of sunspots and starspots, as well as the profound gaps in our understanding of some aspects of their behaviour. Our purpose in this concluding chapter is to identify the major unsolved problems involving the physics of sunspots, starspots and stellar magnetic activity, and to indicate those areas where we expect to see significant progress in the future, as techniques and facilities develop.

It is already apparent that solar observations, from the ground and from space, will continue to achieve higher resolution and increased precision. Meanwhile, stellar observations will attain greater resolution through improved spectroscopy and the introduction of interferometry, and stellar activity cycles will be followed using dedicated telescopes. Theory will also progress, depending not only on physical insight but also on the ever increasing power of high-performance computers, and the possibility of carrying out ever more realistic simulations.

13.1 The structure and dynamics of a sunspot

The Hinode satellite has only recently begun to deliver results, and will continue for a good many years, to be joined by the Solar Dynamics Orbiter in 2009. On the ground, the 1-m Swedish Solar Telescope will continue to provide important new results, and the 1.5-m German GREGOR solar telescope in Tenerife is due to become operational in 2008, to be followed in the USA by the 4-m Advanced Technology Solar Telescope in Hawaii by about 2014. These facilities will refine and extend our knowledge of the structure of a sunspot and the patterns of intensity, velocity and magnetic fields within it.

A detailed discussion of stellar atmospheres is beyond the scope of this book. Nevertheless, our means of studying the properties of hot massive stars relies upon our ability to properly interpret the stellar continuum and line information typically formed in the thin boundary layer between the unseen interior and effectively vacuum interstellar medium. An excellent monograph on the topic of stellar photospheres is provided by Gray (2005), whilst more advanced techniques are introduced by Mihalas (1978).

With respect to normal stars, our interpretation of hot, luminous stars is hindered by two effects. Firstly, the routine assumption of LTE breaks down for high-temperature stars, and particularly for supergiants, due to the intense radiation field, such that the solution of the statistical rate equations (non-LTE) is necessary. Secondly, the simplifying assumption of plane-parallel geometry is no longer valid for blue and red supergiants, so the scale heights of their atmospheres are no longer negligible with respect to their stellar radii. It is the combination of requiring non-LTE plus spherical geometry that has prevented the routine study of OB star atmospheres until recently.

LTE atmospheres

Effective temperatures of early-type stars, essential for subsequent determinations of radii and luminosities, are derived from a comparison between observed photometry and/or spectroscopy and models. Surface gravities also require comparison between observed line profiles and models.

LTE model atmospheres developed by Robert Kurucz during the 1970s and 1980s account very thoroughly for metal line blanketing and are widely employed for both early- and late-type stars.

Sunspots are the most conspicuous but not the only product of solar magnetic activity. In this chapter we relate sunspots to other manifestations of solar activity, such as the emergence of magnetic flux at the solar surface, the formation of active regions, and the organization of magnetic flux into small flux tubes, pores and sunspots. We also consider the evolution of an individual sunspot, from its formation by the coalescence of small magnetic flux tubes and pores to its decay through the loss of magnetic flux at the periphery. Most of this chapter is devoted to features seen in and immediately above the photosphere, but we also attempt to relate these features to processes occurring beneath the solar surface.

Description of active regions

Magnetic activity on the Sun is not uniformly distributed over the solar surface, but instead is concentrated into active regions where sunspots, pores, faculae, plages and filaments are gathered. The underlying cause of all these features of solar activity is the Sun's magnetic field; an active region is basically a portion of the solar surface through which a significant amount of magnetic flux has emerged from the interior. Magnetic fields are also found everywhere on the solar surface outside of active regions, in weaker, more diffuse form or as small flux concentrations in the intergranular lanes, but those fields are not organized into the structures that so clearly define an active region. Active regions can be easily identified in full-disc magnetograms, such as the one shown in Figure 7.1.

Massive stars are born in interstellar clouds made up of molecular gas and dusty material. Most of these stars originate from GMCs with typically ∼105M⊙. Upon collapse, these lead to massive star clusters. Some massive stars are born separate from these massive concentrations of gas and dust in smaller clouds and end up in more compact star clusters. Truly isolated massive stars seem to be rare in our Galaxy. Indeed, de Wit et al. (2005) argue that only a few percent of massive stars are born away from clusters. All these massive stars are found highly concentrated towards the Galactic plane where current star formation is still proceeding, albeit at a relatively restrained rate at present.

Consider first an individual massive star. It is formed when gravitational instability overwhelms a cloudlet of gas and dust which then begins a process of collapse. The collapse brings more and more material to the central object in a process of heating and rapid accretion. This phase is very rapid and will only be observed at far-IR wavelengths as the central material aggregates, begins to heat up, and emits radiation or molecular emission at radio wavelengths. Very quickly sufficient material accumulates such that an individual object can be identified. As this material continues to heat from the continuous contraction and accretion of more gas and dust it takes on the characteristic of what is called a “hot core”, radiating also now at mid-IR wavelengths.

Several textbooks discuss the interstellar medium in depth, notably The Physics of the Interstellar Medium (Dyson & Williams 1997) and The Physics and Chemistry of the Interstellar Medium (Tielens 2005). This chapter focuses on aspects relevant to massive stars, namely the properties of interstellar dust, ionized nebulae surrounding individual O stars (giant HII regions are discussed later on), wind blown bubbles and ejecta nebulae around LBVs and W-R stars.

Diffuse gas in the ISM may be in a neutral or ionized form, of which 90% is in the form of hydrogen, either in an atomic, molecular, or ionized state. Cold (∼100 K), atomic hydrogen can be traced via the 21 cm (1420 MHz) hyperfine line, first predicted by van de Hulst (Bakker & van de Hulst 1945) and observed by Ewen & Purcell (1951). This provided the key means of mapping out the structure of the Milky Way and external galaxies. The Lyman series of neutral hydrogen can also be observed in the UV against a suitably hot background source.

Most of the cold (∼10 K) molecular ISM is in the form of H2. This molecule, however, does not emit at radio wavelengths. Since H2 is well correlated with carbon monoxide (CO), the CO emission at 1.3 and 2.6 mm is used as a proxy for H2.

Over the past four decades, remarkable advances in high-resolution studies of sunspots and active regions have been made through the use of new ground-based telescopes, space missions and associated instrumentation, and of new observing techniques. In this appendix we give a brief summary of these facilities and techniques. (Techniques for observing starspots are described separately in Chapter 9.) An excellent general introduction to modern solar telescopes and their instrumentation is given in Chapter 3 of Stix (2002). References to more thorough treatments of particular topics are provided in the sections below.

A1.1 High-resolution solar telescopes

Although there are more than 50 professional ground-based solar telescopes in regular operation around the world, we focus here only on the few large telescopes that are best suited to high-resolution studies of sunspots. These existing telescopes are listed in Table A1.1 along with two future telescopes, one nearing completion (GREGOR) and the other recently through its design phase and awaiting construction (ATST). There are also plans for a large European Solar Telescope, to come into operation around 2020. A comprehensive list of solar telescopes and their specifications has been compiled by Fleck and Keller (2003).

Ground-based observations are limited to electromagnetic radiation in the visible and near-infrared ranges, at wavelengths between about 300 nm and 2200 nm, and in a range of radio wavelengths. Space missions have allowed us to observe at shorter wavelengths (UV, EUV, X-ray and gamma-ray), revealing the properties of the higher-temperature chromospheric and coronal layers of the solar atmosphere, and have also provided long time series of seeing-free measurements in the visible for studying solar oscillations (helioseismology) and the evolution of solar magnetic fields.