The formation and early evolution of the Milky Way Galaxy

Science

Volumn 287, Issue 5450, 2000, Pages 69-74

  Buser, Roland ^a  

^a UNIVERSITY OF BASEL   (Switzerland)

Author keywords

[No Author keywords available]

Indexed keywords

ASTRONOMY; EVOLUTION; MODEL; MOTION; PHOTOMETRY; PRIORITY JOURNAL; REVIEW; ROTATION; VELOCITY;

ASTRONOMY; COSMIC DUST; EVOLUTION, PLANETARY; HYDROGEN; IRON;

EID: 0034614681     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.287.5450.69     Document Type: Review

References (100)

1. A recent review of the Local Group is given by S. van den Bergh [Astron. Astrophys. Rev. 9, 273 (1999)].
2. 3 pc.
3. R. A. Ibata, G. Gilmore, M. J. Irwin, Nature 370, 194 (1994).
4. J. Bland-Hawthorn and K. Freeman, Science 287, 79 (2000);
5. F. Yusef-Zadeh, F. Melia, M. Wardle, Science 287, 85 (2000)].
6. 9 years, or 10 Gy - whereas some other galaxies have been observed, which have relatively younger globular clusters. For a recent review, see D. Bulgarella [Science 276, 1370 (1997)].
7. ⊙) than globular clusters. Accordingly, they are less centrally concentrated but cover a wide range between loosely and densely packed. Because they are preferentially found in and near the Galactic disk, they are also called Galactic clusters.
8. The geometric figures assigned to the Galaxy's components - bar-like bulge, flat disks, and spheroidal halo - can be and also have been inspired by our outsider view of extragalactic systems, some of which we happen to see from the same edge-on perspective as we have of the Milky Way from our insider's vantage point close to the mid-plane of the Galactic disk, about 8.5 kpc from the Galactic center. However, in contrast to an external galaxy whose morphology may be immediately recognized on a single photograph imaging a small solid angle in the sky, a quantitative description of the Galaxy's large-scale morphological structure obviously requires the more indirect and difficult approach of combining many partial surveys of its individual components, distributed over the full sky.
9. See also F. D. A. Hartwick [in Unsolved Problems of the Milky Way, L. Blitz and P. Teuben, Eds. IAU Symp. 169 (Kluwer, Dordrecht, Netherlands, 1996), p. 669] for a concise description of the Milky Way's position and extent in the hierarchical structure of the local universe.
10. [B. Battrick, Ed., HIPPARCOS Venice '97, ESA Spec. Publ. No. 402, Noordwijk NL: ESA Publ. Div. ESTEC, 52+862pp. (1997)]. Although results first appeared to indicate that current theoretical understanding of stellar luminosities in clusters was more seriously incomplete than had been known [cf. J. Kovalevsky, Annu. Rev. Astron. Astrophys. 36, 99 (1998) and references therein],
11. [B. Battrick, Ed., HIPPARCOS Venice '97, ESA Spec. Publ. No. 402, Noordwijk NL: ESA Publ. Div. ESTEC, 52+862pp. (1997)]. Although results first appeared to indicate that current theoretical understanding of stellar luminosities in clusters was more seriously incomplete than had been known [cf. J. Kovalevsky, Annu. Rev. Astron. Astrophys. 36, 99 (1998) and references therein], Although results first appeared to indicate that current theoretical understanding of stellar luminosities in clusters was more seriously incomplete than had been known [cf. J. Kovalevsky, Annu. Rev. Astron. Astrophys. 36, 99 (1998) and references therein],
12. more detailed analyses have dispelled much of these concerns [for example, D. Egret and A. Heck, Eds., Harmonizing Cosmic Distance Scales in a Post-Hipparcos Era, Astron. Soc. Pacific Conf. Series, vol. 167 (1999)].
13. Thus, we may still say that cluster distances, derived from observations of a large number of member stars in each cluster, are among the most reliable and accurate distances that can be determined at all [see also (15)].
14. Two components of a star's velocity can be directly obtained from observations: the radial velocity - along the line of sight - is determined (in kilometers per second) by measuring the Doppler shifts of atomic lines in the stellar spectra, while the tangential velocity - perpendicular to the line of sight - is derived (in kilometers per second) by combining the star's distance with its annual angular displacement, or proper motion, on the sky [measured in arcseconds per year from photographic or CCD (charge-coupled device) surveys taken at different epochs]. In general, determination of a third velocity component and, hence, of the full space velocity vector, requires calculations based on an assumed gravitational potential of the Galaxy. Note that for many purposes, available velocity data of only one kind - either radial velocities or proper motions - may suffice.
15. C. Alcock, Science 287, 74 (2000).
16. A comprehensive recent account of current work on all aspects of star formation, stellar evolution, and the final stages in stellar lives appeared in the special issue on Stellar Birth and Death, Science 276 (30 May 1997), pp. 1350-1391.
17. i/Fe] in stars were seen to have values ≈ 0 throughout the observed Fe-abundance range (that is, for +1 > [Fe/H] ≳ -3) and all metal-abundance differences between stars could be described as varying in lockstep with the Fe abundance difference: stars exhibit a cosmic abundance distribution, and [Fe/H] can be commonly used as a representative measure of stellar metallicity, [M/H]. Typically, [M/H] = [Fe/H] = 0 for a metal-rich star and [M/H] = [Fe/H] = -2 for a metal-poor star. As abundance determinations became more accurate, because of improved instruments and measuring techniques developed since about 1970, observed variations of elemental abundances in stars were soon understood as sensitive diagnostics of stellar-interior nuclear processes and mixing of products into the atmospheres during the different evolutionary stages.
18. For recent reviews, see A. McWilliam, Annu. Rev. Astron. Astrophys. 35, 503 (1997);
19. i/Fe] in stars were seen to have values ≈ 0 throughout the observed Fe-abundance range (that is, for +1 > [Fe/H] ≳ -3) and all metal-abundance differences between stars could be described as varying in lockstep with the Fe abundance difference: stars exhibit a cosmic abundance distribution, and [Fe/H] can be commonly used as a representative measure of stellar metallicity, [M/H]. Typically, [M/H] = [Fe/H] = 0 for a metal-rich star and [M/H] = [Fe/H] = -2 for a metal-poor star. As abundance determinations became more accurate, because of improved instruments and measuring techniques developed since about 1970, observed variations of elemental abundances in stars were soon understood as sensitive diagnostics of stellar-interior nuclear processes and mixing of products into the atmospheres during the different evolutionary stages. D. Arnett and C. Bazan, Science 276, 1359 (1997). For recent reviews, see A. McWilliam, Annu. Rev. Astron. Astrophys. 35, 503 (1997); D. Arnett and C. Bazan, Science 276, 1359 (1997).
20. eff ≥= ∼4000 K, which make up the bulk of Galactic field stars, this slope is very sensitive to metallicity, due to crowding of Fe-peak absorption lines in the U-bands [for example, R. Buser and R. L. Kurucz, Astron. Astrophys. 264, 557 (1992)]. Even though photometry-based metallicities are less accurate than spectroscopic abundances, they can be accumulated relatively easily for large samples of faint stars, providing indispensable complementary data of high statistical weight from large-scale field surveys of the Galactic components.
21. eff), the fundamental diagram of astrophysics.
22. The most direct way to derive the age of a cluster is by fitting the so-called turnoff (TO) luminosity and color (where cluster stars are turning away from the main-sequence due to exhaustion of core hydrogen and ensuing evolution to the red-giant stage) in the observed CMD with a grid of theoretical isochrones (that is, same-age loci) calculated for model stars of appropriate metallicity and for a representative age range. There are a number of other morphological features in the cluster CMDs, which are suitable for age determinations, such as the horizontal branch (HB) or the luminosity difference between the HB and the TO [for a recent review of the most promising methods, see A. Sarajedini, B. Chaboyer, P. Demarque, Publ. Astron. Soc. Pacific 109, 1321 (1997)],
23. but in principle, all stellar evolutionary stages sampled in the observed CMD of a given cluster should be consistent, to within the accuracy of the data, with the same theoretical isochrone. Although currently available theoretical models cannot yet account for the full morphological variety and details exhibited in cluster CMDs, and although uncertainties in individual derived absolute cluster ages are accordingly large (∼3 to 5 Gy), relative ages obtained from differential studies of cluster samples are among the most accurate (∼±2 Gy) presently available. An authoritative review of the current state of the art recently appeared [D. A. VandenBerg, M. Bolte, P. B. Stetson, Annu. Rev. Astron. Astrophys. 34, 461 (1996)].
24. The famous Vatican Conference in 1957 was devoted to Stellar Populations [J. O'Connell, Ed., Stellar Populations, Specola Astronomica Vaticana, vol. 5 (1958)], ^10m^a concept of integrating the properties and relationships among the different stellar components in galaxies, which was introduced by the far-sighted Walter Baade, following his account [Astrophys. J. 100, 137 (1944)] of the CMD of stars that he had been able to resolve, for the first time ever, in the Andromeda nebula. but in principle, all stellar evolutionary stages sampled in the observed CMD of a given cluster should be consistent, to within the accuracy of the data, with the same theoretical isochrone. Although currently available theoretical models cannot yet account for the full morphological variety and details exhibited in cluster CMDs, and although uncertainties in individual derived absolute cluster ages are accordingly large (∼3 to 5 Gy), relative ages obtained from differential studies of cluster samples are among the most accurate (∼±2 Gy) presently available. An authoritative review of the current state of the art recently appeared [D. A. VandenBerg, M. Bolte, P. B. Stetson, Annu. Rev. Astron. Astrophys. 34, 461 (1996)].
25. The famous Vatican Conference in 1957 was devoted to Stellar Populations [J. O'Connell, Ed., Stellar Populations, Specola Astronomica Vaticana, vol. 5 (1958)], ^10m^a concept of integrating the properties and relationships among the different stellar components in galaxies, which was introduced by the far-sighted Walter Baade, following his account [Astrophys. J. 100, 137 (1944)] of the CMD of stars that he had been able to resolve, for the first time ever, in the Andromeda nebula. but in principle, all stellar evolutionary stages sampled in the observed CMD of a given cluster should be consistent, to within the accuracy of the data, with the same theoretical isochrone. Although currently available theoretical models cannot yet account for the full morphological variety and details exhibited in cluster CMDs, and although uncertainties in individual derived absolute cluster ages are accordingly large (∼3 to 5 Gy), relative ages obtained from differential studies of cluster samples are among the most accurate (∼±2 Gy) presently available. An authoritative review of the current state of the art recently appeared [D. A. VandenBerg, M. Bolte, P. B. Stetson, Annu. Rev. Astron. Astrophys. 34, 461 (1996)].
26. A. Blaauw and M. Schmidt, Eds., Galactic Structure (Univ. of Chicago Press, Chicago, 1965), in particular, the article entitled The Concept of Stellar Populations by A. Blaauw, p. 435. Although Blaauw's account has been expanded and some of its details have been superseded by now, its basic message on the contents and structure of the large-scale galaxy has proven surprisingly robust. 18. R. B. C. Henry and G. Worthey, Publ. Astron. Soc. Pacific 111, 919 (1999), and references therein. This excellent review entitled The Distribution of Heavy Elements in Spiral and Elliptical Galaxies also provides a pertinent discussion of the relevant data for the Milky Way.
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100. Supported by the Swiss National Science Foundation. I would like to thank J. X. Rong for discussions and D. Cerrito for expert help with the preparation of the figures.