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Star Formation and Mass Assembly in High Redshift Galaxies

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A&A 504, 751–767 (2009)

DOI: 10.1051/0004-6361/200811434

Astronomy

&

  • c
  • ꢀ ESO 2009

Astrophysics

Star formation and mass assembly in high redshift galaxiesꢀ

P. Santini1,2, A. Fontana1, A. Grazian1, S. Salimbeni1,3, F. Fiore1, F. Fontanot4, K. Boutsia1, M. Castellano1,2
S. Cristiani5, C. De Santis6,7, S. Gallozzi1, E. Giallongo1, N. Menci1, M. Nonino5, D. Paris1,
L. Pentericci1, and E. Vanzella5
,

1

INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio (RM), Italy

e-mail: [email protected]

Dipartimento di Fisica, Università di Roma “La Sapienza”, P.le A. Moro 2, 00185 Roma, Italy Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003, USA MPIA Max-Planck-Institute für Astronomie, Koenigstuhl 17, 69117 Heidelberg, Germany INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy Dip. di Fisica, Università Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy INFN – Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy

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Received 27 November 2008 / Accepted 24 April 2009

ABSTRACT

Aims. The goal of this work is to infer the star formation properties and the mass assembly process of high redshift (0.3 ≤ z < 2.5) galaxies from their IR emission using the 24 μm band of MIPS-Spitzer. Methods. We used an updated version of the GOODS-MUSIC catalog, which has multiwavelength coverage from 0.3 to 24 μm and either spectroscopic or accurate photometric redshifts. We describe how the catalog has been extended by the addition of mid-IR fluxes derived from the MIPS 24 μm image. We compared two different estimators of the star formation rate (SFR hereafter). One is the total infrared emission derived from 24 μm, estimated using both synthetic and empirical IR templates. The other one is a multiwavelength fit to the full galaxy SED, which automatically accounts for dust reddening and age-star formation activity degeneracies. For both estimates, we computed the SFR density and the specific SFR. Results. We show that the two SFR indicators are roughly consistent, once the uncertainties involved are taken into account. However, they show a systematic trend, IR-based estimates exceeding the fit-based ones as the star formation rate increases. With this new catalog, we show that: a) at z > 0.3, the star formation rate is correlated well with stellar mass, and this relationship seems to steepen with redshift if one relies on IR-based estimates of the SFR; b) the contribution to the global SFRD by massive galaxies increases with redshift up to ꢁ2.5, more rapidly than for galaxies of lower mass, but appears to flatten at higher z; c) despite this increase, the most important contributors to the SFRD at any z are galaxies of about, or immediately lower than, the characteristic stellar mass; d) at z ꢁ 2, massive galaxies are actively star-forming, with a median SFR ꢁ 300 M yr−1. During this epoch, our targeted galaxies

ꢂ

assemble a substantial part of their final stellar mass; e) the specific SFR (SSFR) shows a clear bimodal distribution. Conclusions. The analysis of the SFR density and the SSFR seems to support the downsizing scenario, according to which high mass galaxies have formed their stars earlier and more rapidly than their low mass counterparts. A comparison with renditions of theoretical simulations of galaxy formation and evolution indicates that these models follow the global increase in the SSFR with redshift and predict the existence of quiescent galaxies even at z > 1.5. However, the average SSFR is systematically underpredicted by all models considered.

Key words. galaxies: evolution – galaxies: high-redshift – galaxies: fundamental parameters – galaxies: photometry – galaxies: starburst

1. Introduction

Rudnick et al. 2006; Papovich et al. 2006; Yan et al. 2006;

Pozzetti et al. 2007) and demonstrated that a substantial fraction (30–50%) of the stellar mass formed during this epoch. The differential evolution in the galaxy stellar mass function, according to which massive galaxies evolve rapidly up to z ∼ 1.5 and then more gradually until the present epoch, while less massive galaxies continue to evolve, implies that massive galaxies must have already formed by z ∼ 1.5. Several groups (e.g., Faber et al. 2007; Brown et al. 2007) studied the evolution of massive galaxies at z ꢀ 1, and their migration from the blue cloud to the red sequence. We note that optical observations (Bell et al. 2004; Zucca et al. 2006) suggest that the number of massive galaxies, as well as the stellar mass on the red sequence, has nearly doubled since z ∼ 1, in qualitative agreement with the hierarchical merging scenario.
Answering the basic questions about the birth, formation, mass build-up, and evolution of galaxies throughout cosmic time are some of the major goals of observational extragalactic astronomy.
In the past few years, this issue has been approached with two different methods. Many previous works measured a rapid evolution in the stellar mass density between z ∼ 1 and

z ∼ 3 (Dickinson et al. 2003; Fontana et al. 2003, 2004; Glazebrook et al. 2004; Drory et al. 2004; Fontana et al. 2006;

ꢀ

GOODS-MUSIC multiwavelength photometric catalog is available in electronic form at the CDS via anonymous ftp to

cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/504/751

Article published by EDP Sciences

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  • P. Santini et al.: Star formation and mass assembly in high redshift galaxies

A parallel line of study has analyzed the rate at which galax- The total infrared luminosity is generally estimated by comies form stars during different epochs and shown that they expe- paring observed SEDs and synthetic templates, although empiririence an extremely active phase in the same redshift range (e.g., cal conversions have sometimes been used (Takeuchi et al. 2005;

Lilly et al. 1996; Madau et al. 1996; Steidel et al. 1999; Hopkins Bavouzet et al. 2008). A variety of different libraries are used 2004; Hopkins & Beacom 2006; Daddi et al. 2007b). Galaxies for this purpose (e.g., Chary & Elbaz 2001; Dale & Helou 2002;

appear to form their stars following the so-called downsizing sce- Lagache et al. 2003; Siebenmorgen & Krügel 2007, and so on). nario, in which the star formation shifts from high mass to low A notable problem for the reliability of IR-based SFR tracers mass galaxies as redshift decreases. This picture was first intro- concerns obscured AGNs. In these objects the IR emission is duced by Cowie et al. (1996), who studied the evolution in the generated by matter accretion onto a central black hole rather Ks band luminosity function with redshift, and proposes that the than dust heating by young stars. most massive galaxies assemble their mass both earlier and more quickly than their lower mass counterparts, which, in contrast, continue to form stars until recent epochs. Later on, many other

groups (e.g., Brinchmann & Ellis 2000; Fontana et al. 2003; Feulner et al. 2005; Pérez-González et al. 2005; Papovich et al.

2006; Damen et al. 2009) derived confirmations of a downsiz- ing behaviour from the study of the specific star formation rate, defined to be the star formation rate per unit mass, at different redshifts. However, we note that deep radio observations (Dunne et al. 2009) appear to conflict with this scenario. The downsizing picture also appears to contradict the hierarchical growth scenario in which the most massive structures that we see today are produced by merging processes between smaller structures inside large-scale overdensities and collapsed when the Universe was far younger than today.
In this paper, we use the GOODS-MUSIC catalog to investigate properties of star-forming galaxies up to redshift 2.5 and infer the mass assembly process from their mid-IR emission. The paper is organized as follows. In Sect. 2, we recall the basic features of the GOODS-MUSIC dataset and explain the innovations concerning its latest version, and we explain how it has been updated with the addition of the 24 μm photometric band. We derive and compare star formation rates from IR- and fit-based estimators in Sect. 3. In Sect. 4, we present a study of the mass assembly process in the high redshift Universe and a comparison with theoretical model predictions. We finally summarize our work and our conclusions in Sect. 5. In Appendix A, we describe in more detail how we convert mid-IR fluxes into total infrared luminosities and compare the different templates used, and in Appendix B we present the error analysis performed on the fit-based SFR estimates.
To reproduce this early formation of massive galaxies (see

Thomas et al. 2005) that are already “red and dead” at high z, theoretical models had to introduce very efficient processes of star formation and its suppression by means of active galactic nuclei and supernovae quenching of cooling flows (Menci et al.

2006; Kitzbichler & White 2007; Bower et al. 2006; Croton et al. 2006; Nagamine et al. 2006; Monaco et al. 2007), gravitational heating (Khochfar & Ostriker 2008; Johansson et al. 2009), or

shock heating (Dekel & Birnboim 2006). These models differ slightly in their predictions mainly because they adopt different processes to shut down the star formation.
Throughout this work, unless stated otherwise, we assume a Salpeter (1955) initial mass function (IMF) and adopt the Λ-CDM concordance cosmological model (H0

=

70 km s−1 Mpc−1, ΩM = 0.3 and ΩΛ = 0.7).

2. The data sample

2.1. The new GOODS-MUSIC sample

We present and use an updated version of the multicolour GOODS-MUSIC sample (GOODS MUlticolour Southern Infrared Catalog, Grazian et al. 2006), extracted from the public data of the GOODS-South survey (Giavalisco et al. 2004). In the following, we shall refer to this version of the catalog as GOODS-MUSIC v2, to differentiate it from the former public version, which is named v1 hereafter. The new version is also made publicly available1.
The 15-bands multiwavelength coverage ranges from 0.35 to 24 μm, as a result of the combination of images from different instruments (2.2ESO, VLT-VIMOS, ACS-HST, VLT- ISAAC, Spitzer-IRAC, Spitzer-MIPS). The catalog covers an area of ∼143.2 arcmin2 located in the Chandra Deep Field South and consists of 15 208 sources. After culling Galactic stars, it contains 14 999 objects selected in either the z band or the Ks band or at 4.5 μm.
The whole catalog has been cross-correlated with spectroscopic catalogs available to date, and a spectroscopic redshift has been assigned to ∼12% of all sources. For all other objects, we have computed well-calibrated photometric redshifts using a standard χ2 minimization technique for a large set of synthetic spectral templates.
The previous version of the catalog and procedures adopted to determine the photometric redshifts and physical properties of each object were described at length in Grazian et al. (2006)
Both stellar masses and star formation rate estimates are affected by a number of uncertainties. The measure of the star formation rate (SFR) is especially difficult to handle. The high amount of energy produced by newly born stars is emitted throughout the galaxy spectral energy distribution (SED), from X-rays to radio frequencies. Since we are of course unable to directly measure the total light emitted by young and massive stars, calibration factors and corrections are applied to estimate its value for any of these frequency ranges (Kennicutt 1998; Bell 2003; Calzetti 2008). One of the most commonly used estimators is the UV rest-frame band, where young and massive stars emit most of their light. However, dust absorbs, reprocesses, and re-radiates UV photons at near-to-far IR wavelengths. Hence, the reliability of UV luminosity as a SFR tracer depends on large and uncertain corrections relying upon the dust properties, which are

not yet clearly known (Calzetti et al. 1994; Calzetti 1997, 2001).

Moreover, the UV-upturn at λ shortward of 2500 Å (e.g., Han et al. 2007), especially in elliptical galaxies, can potentially bias the SFR estimate at very low redshift. Since the most intense star formation episodes are expected to occur in dusty regions, most of the power originating in star-forming (SF) galaxies is emitted in this wavelength range, and the dust emission peak is the dominant component of SF galaxies SEDs (Adelberger

& Steidel 2000; Calzetti et al. 2000). Thus, a popular approach

consists of adopting a conversion between the total emitted IR luminosity (LIR hereafter) and a star formation rate estimation that is unaffected by dust obscuration (Kennicutt 1998).

1

The catalog is available in electronic form at the CDS. It is also possible to download the catalog at the WEB site http://lbc.mporzio.

astro.it/goods.

  • P. Santini et al.: Star formation and mass assembly in high redshift galaxies
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and Fontana et al. (2006). With respect to the previous catalog, we have performed a set of improvements to the optical-near-IR data, the major ones being:
For the latter, we have first removed all objects whose spectra show AGN features. Then, we have cross-correlated our catalog with the X-ray catalog of Brusa et al. (2009), and removed all X-ray detected sources whose flux is dominated by an unresolved central source. These sources typically have spectra classified as narrow-line AGNs. The optical morphologies of all remaining X-ray sources do not show a dominating central point like source, and – where available – have typically spectra classified as emission-line starforming galaxies. These objects have been retained in our galaxy sample.
– In addition to objects selected in the ACS z and in the ISAAC
Ks bands, we have also included objects selected from the IRAC 4.5 μm image, hence including sources detected at 4.5 μm but very faint or undetected even in Ks band. A full description of these objects is beyond the scope of the present paper and will be presented elsewhere.
– We have revised the photometry in the four IRAC bands using an updated version of the PSF-matching kernels, as released by the Spitzer Science Center. As a consequence, we also adopted a larger kernel, to fully account for the large tails of the IRAC PSFs.
– We have adopted a revised procedure for estimating the background in the IRAC images. Using the objects’ positions and IRAC fluxes from the GOODS-MUSIC v1 catalog, we have created realistic simulated images in the four IRAC bands by smoothing sources to the nominal IRAC PSFs. An accurate background estimation has been performed by subtracting these simulated images from the original ones and by linearly interpolating the residual emission. Since the average value of the apparent IRAC background is negative, this has led to an increase in the adopted background, with respect to the GOODS-MUSIC v1 version.
– These two changes have modified the IRAC photometry.
Because of the new kernels, the brightest objects have a higher flux and a typical offset in magnitude of 0.23, 0.14, 0.22, 0.35, respectively in the 3.6, 4.5, 5.8, and 8 μm images, with respect to the GOODS-MUSIC v1 catalog. The new photometry agrees with the analysis of Wuyts et al. (2008). This effect is largely mitigated for fainter galaxies, since the higher background now adopted has led to an underestimate of their fluxes.
The major new ingredient of this new version of the GOODS- MUSIC catalog, however, is the inclusion of the 24 μm photometry for all galaxies in the sample, which is the main focus of the present paper. We describe the adopted procedures and results in the following.

2.2. MIPS 24 μm catalog

We have extended the GOODS-MUSIC catalog with the addition of the mid-IR fluxes derived from the public 24 μm image of the Multiband Imager Photometer for Spitzer (MIPS, Rieke et al. 2004) onboard the Spitzer Space Telescope. As for the IRAC images, given the very large PSF of this image (∼5.2 arcsec), to properly detect and de-blend objects we had to employ a PSF-matching technique, which is performed by the software ConvPhot (De Santis et al. 2007). This algorithm measures colours from two images of different qualities by exploiting the spatial and morphological information contained in the higher resolution image.
When applying ConvPhot to our case, each object was extracted from the high resolution z band ACS-HST (PSF ∼ 0.12 ) image, which was used as a prior to extract the objects’ positions, filtered with a convolution kernel, and finally scaled by a χ2 minimization over all the image pixels to match the intensity in the MIPS image. To fully use the positional information of the ACS images, and maintain consistency with the z-selected sample, we used the z band ACS-HST data as a prior, augmented by artificial objects placed where galaxies detected only in Ks or 4.5 μm were located. In our case, the MIPS-Spitzer and ACS- HST images have pixel scales of 1.2 and 0.03 arcsec/pixel, which means that it is impracticable to use ConvPhot even with fast workstations. To make the computation feasible, we rebinned the ACS detection image by a factor 8 × 8 (0.24 arcsec/pixel). In regions where the crowding of the z-detected sources was signifi- cant, the fit may become unconstrained or degenerate because of the large size of the MIPS PSF. To prevent this, we placed an additional constraint on the fitted fluxes that must be non-negative. For the objects whose flux is forced to be zero, we provide an upper limit derived from the analysis of the mean rms in the object area.
– Overall, the revised IRAC photometry has a modest impact on the estimate of photometric redshifts, since ꢃzphot v2

−

zphot v1ꢄ ∼0.01 0.16 for the whole sample and ∼0.01 0.03 when a 3σ-clipping analysis is performed (see also Wuyts

et al. 2008).

– A more informative test of the photometric redshift accuracy comes from the enlargement of the sample of galaxies with spectroscopic redshifts, which we obtained by adding new spectra from public surveys (Vanzella et al. 2008; Popesso et al. 2009). In addition, we have also had access to the spectra of the GMASS survey (Cimatti et al. 2008), prior to their publication. The final sample now includes 1888 galaxies, three times larger than the spectroscopic sample in Grazian et al. (2006). The additional spectra are mostly relative to galaxies that are both fainter and at higher redshift than in the original sample. Without significant refinements in the adopted templates, we then find that the absolute scat-

ter |Δz| = |zspe − zphot|/(1 + zspe) has a slightly larger aver-

age value. Quantitatively, the average absolute scatter is now ꢃ|Δz|ꢄ = 0.06, instead of 0.045 obtained for the GOODS- MUSIC v1 catalog. However, when only the brightest galaxies are considered, we find comparable values with respect to Grazian et al. (2006) (ꢃ|Δz|ꢄ = 0.043). We have verified that this is due to an increased number of outliers, as shown by a 3σ-clipping analysis, which provides ꢃ|Δz|ꢄ = 0.027
The behaviour of ConvPhot in these extreme applications was tested with several simulations, which were described by De Santis et al. (2007). These tests indicated that the estimated magnitudes are biased by neither the different qualities of the two images nor the undersampling of the high resolution image after rebinning.
Finally, from visual inspection of sources with unusual and 0.032 for the complete datasets of v1 and v2 catalogs, colours, we removed ∼30 objects from the catalog whose flux respectively. was incorrectly assigned. By examination of the residuals, we
– We have removed Galactic stars and performed a more care- also verified that there is no significantly bright source in the ful selection of the galaxy sample to identify AGN sources. MIPS image apart from the ones considered. We detected 3313

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agree with those estimated by an independent detection with SExtractor for sources that are not severely blended. For the blended fraction of the objects, however, the fluxes estimated by ConvPhot can be slightly lower, since part of the flux is identified with the fainter contaminants, which are not detected in the 24 μm image alone.

1000
100
10

It is more interesting to consider the behaviour at faint fluxes, where the a priori knowledge of the object position, because of the use of the z, Ks and 4.5 μm images for the detection, allows us to obtain flux estimate at much fainter limits, reducing the effects of blending and confusion. As expected, indeed, SExtractor counts decline at ∼100 μJy, where the confusion limit prevents the detection of fainter sources, while ConvPhot allows us to complete our detection to even deeper limits. ConvPhot number counts present a double slope, with a break point located at ∼100 μJy, which we consider to be an intrinsic property of the sample. The slope and the normalization at the faint end agree with the estimates of Papovich et al. (2004), who carefully computed a correction for the incompleteness due to poor resolution at faint limits. Papovich et al. (2004) and subsequent papers (e.g.,

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    Star Classification Stars are classified by their spectra (the elements that they absorb) and their temperature. There are seven main types of stars. In order of decreasing temperature, O, B, A, F, G, K, and M. O and B stars are uncommon but very bright; M stars are common but dim.. An easy mnemonic for remembering these is: "Oh be a fine girl, kiss me." Hertzsprung - Russell Diagram The Hertzsprung -Russell (H-R) Diagram is a graph that plots stars color (spectral type or surface temperature) vs. its luminosity (intrinsic brightness or absolutemagnitude). On it, astronomers plot stars' color, temperature, luminosity, spectral type, and evolutionary stage. This diagram shows that there are 3 very different types of stars: Most stars, including the sun, are "main sequence stars," fueled by nuclear fusion converting hydrogen into helium. For these stars, the hotter they are, the brighter. These stars are in the most stable part of their existence; this stage generally lasts for about 5 billion years. As stars begin to die, they become giants and supergiants (above the main sequence). These stars have depleted their hydrogen supply and are very old. The core contracts as the outer layers expand. These stars will eventually explode (becoming a planetary nebula or supernova, depending on their mass) and then become white dwarfs, neutron stars, or black holes (again depending on their mass). Smaller stars (like our Sun) eventually become faint white dwarfs (hot, white, dim stars) that are below the main sequence. These hot, shrinking stars have depleted their nuclear fuels and will eventually become cold, dark, black dwarfs.
  • Are the Bulge C-Stars in the Sagittarius Dwarf Galaxy?

    Are the Bulge C-Stars in the Sagittarius Dwarf Galaxy?

    OSSERVATORIO ASTRONOMICO di PADOVA 24.9.2018 Are the Bulge C – stars in the Sagittarius dwarf galaxy? Yuen K. Ng Padova Astronomical Observatory, Vicolo dell’Osservatorio 5, I-35122 Padua, Italy ([email protected]) Received 21 March 1997 / Accepted 10 July 1997 Abstract. Part of the mystery around the Bulge carbon (Azzopardi et al. 1991 – hereafter referred to as ALRW91, stars from Azzopardi et al. (1991) is solved, if they are re- Tyson & Rich 1991, Westerlund et al. 1991). These stars lated to the Sagittarius dwarf galaxy. The carbon stars are show similarities with the low- to medium bolometric lu- in that case not metal-rich as previously thought, but they minosity SMC carbon stars, but the galactic carbon stars have a metallicity comparable to the LMC, with an age be- have stronger NaD doublets. Various studies suggest that tween 0.1–1 Gyr. A significant fraction of the carbon stars a wide metallicity range is present in the bulge (Whit- still have luminosities fainter than the lower LMC limit of ford & Rich 1983, Rich 1988 & 1990, Geisler et al. 1992, m Mbol ≃ –3 . 5. A similar trend is present among some of McWilliam & Rich 1994, Ng 1994, Bertelli et al. 1995, the carbon stars found in other dwarf spheroidals, but Ng et al. 1995 & 1996, Sadler et al. 1996). According to m they do not reach a limit as faint as Mbol ≃ –1 . 4 found ALR88 the carbon stars are expected to be metal-rich if for the SMC. At present, the TP-AGB models cannot ex- they belong to the Bulge.
  • 1992Aj 104. . 203W the Astronomical Journal

    1992Aj 104. . 203W the Astronomical Journal

    203W . THE ASTRONOMICAL JOURNAL VOLUME 104, NUMBER 1 JULY 1992 104. STUDIES OF HOT B SUBDWARFS. VIII. STRÖMGREN PHOTOMETRY OF HOT, HYDROGEN-RICH SUBDWARF CANDIDATES IN THE PALOMAR-GREEN AND KITT PEAK-DOWNES SURVEYS 1992AJ F. Wesemael,1 G. Fontaine,1 P. Bergeron, and R. Lamontagne Département de Physique, Université de Montréal, C. P. 6128, Suce. A, Montréal, Québec H3C 3J7, Canada R. F. Green Kitt Peak National Observatory, National Optical Astronomy Observatories, P.O. Box 26732, Tucson, Arizona 85726-6732 Received 5 March 1992 ABSTRACT We present the results of an extensive program of photoelectric observations on the Strömgren system of 310 hot, hydrogen-rich (B and OB) subdwarf candidates in the Palomar-Green and Kitt Peak- Downes colorimetric surveys. 1. INTRODUCTION able information on the importance of this channel to the white dwarf stage (Drilling & Schönbemer 1985; Heber Colorimetric surveys, such as the Palomar-Haro- 1986), and on whether or not these stars are true disk Luyten survey (Haro & Luyten 1962), the Palomar-Green objects (e.g., Baschek & Norris 1975). survey (Green et al 1986), the Kitt Peak-Downes survey With these considerations in mind, we have carried out (Downes 1986), the Kiso survey (Noguchi et al 1980; extensive photoelectric observations in the Strömgren sys- Kondo et al 1984), and the ongoing Edinburgh-Cape tem of a large, magnitude-limited sample of subdwarf stars (Stobie et al 1987; Kilkenny et al 1991) and Montréal- identified in the PG survey. These observations represent a Cambridge-Tololo (Demers et al 1986) surveys are a significant extension of those reported by Bergeron et al valuable source of new, blue subluminous objects, above all (1984; hereafter referred to as Paper I), where Strömgren hot subdwarfs of different kinds.
  • Variable Star

    Variable Star

    Variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either: Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks. Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes Trifid Nebula contains Cepheid variable stars eclipses it. Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.[1] Contents Discovery Detecting variability Variable star observations Interpretation of observations Nomenclature Classification Intrinsic variable stars Pulsating variable stars Eruptive variable stars Cataclysmic or explosive variable stars Extrinsic variable stars Rotating variable stars Eclipsing binaries Planetary transits See also References External links Discovery An ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago may be the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol.[2][3][4] Of the modern astronomers, the first variable star was identified in 1638 when Johannes Holwarda noticed that Omicron Ceti (later named Mira) pulsated in a cycle taking 11 months; the star had previously been described as a nova by David Fabricius in 1596. This discovery, combined with supernovae observed in 1572 and 1604, proved that the starry sky was not eternally invariable as Aristotle and other ancient philosophers had taught.
  • Stellar Evolution

    Stellar Evolution

    Stellar Evolution Sun-like Stars Massive Stars Evidence from Star Clusters Binary Stars The Life of Main-Sequence Stars . Stars gradually exhaust their hydrogen fuel. In the process of aging, they gradually become brighter and a little cooler. They evolve from zero-age main sequence (ZAMS) moving up and slightly to the right on the H-R diagram. Evolution of a Sun-like Star Even while on the main sequence, the composition of a star’s core is changing. Hydrogen decreases while helium increases. Evolution of a Sun-like Star . As the fuel in the core is used up, the core contracts . When the fuel is used up the core begins to collapse because the source of thermal pressure is no longer there. Hydrogen begins to fuse outside the core in a shell, which is called hydrogen shell burning. Evolution off the Main Sequence: Expansion into a Red Giant . Hydrogen in the core is completely converted into He: “Hydrogen burning” (i.e. fusion of H into He) ceases in the core. H burning continues in a shell around the core. He core + H-burning shell produces heat that increases pressure causing the mass above the shell to expand Expansion and cooling of the outer layers of the star Red Giant Expansion onto the Giant Branch . Expansion and surface cooling during the phase of an inactive He core and a H-burning shell Sun will expand beyond Earth’s orbit! Degenerate Matter . Matter in the He core has no energy source left. Not enough thermal pressure to resist and to balance gravity even though the core heats up from gravitational energy.